So, I have allmenus enabled, which means testingcheats is supposed to be permanently enabled. When I had first enabled allmenus, testingcheats was indeed on. However after some months, apparently it got... Disabled? I had only just disabled it myself a few times, but the next play session it was on, just as it should. Now, I don't know what happened exactly, but at some point I opened the game and I noticed debug mode could not open in CAS, and then noticed overall testingcheats functions were missing. In other words, disabled and I now have to open testingcheats myself first thing I do when I open the game. I tested if the allmenus cheat works and yes, it indeed works, and the globalprops file is the updated one I edited to enable allmenus.

Anyone have any ideas? Allmenus even works, only testingcheats isnt always on for some reason. This has been going on for a few months and I honestly had no idea how to fix it, so I decided to turn here for help. I need testingcheats to know what's going on if an object error pops up and it's helpful in general. I really dont want to have a userstartup cheat file because I already have allmenus on and testingcheats is the only cheat I need on for each play session.

Check that you don't have a userstartup.cheat file with the testingcheats disabled in it. SimPE for example has a check box to start your game in debug mode or not.

I haven't created any userstartup cheat file, actually.

What is the reason why you want to avoid userStartup.cheat and turn on debug mode there? It is a tiny file that does no harm.

Because I would only need it for testingcheats, and allmenus already does enable debug mode at startup. If there is a way to fix allmenus' setting, then I would use that. If there isn't, I'll resort to the userstartup cheat file.

The allmenus cheat requires that the game is booted into debug mode, so the testingcheats must be enabled through the globalprops file. Therefore, if allmenus works, then the testingcheats must have been enabled on boot, so for some reason it is being disabled through a process that is occurring later than boot.
https://sims.fandom.com/wiki/Game_guide:Allmenus_cheat

@FuryCat I'm positing that as your game has been correctly booted into debug, then there must be something in your userdata that is preventing the game from retaining the testingcheats enabled mode.
What the logical thing to try would be is to rename your Users\[username]\Documents\EA Games\The Sims 2 folder and run the game to create a new one. Enter CAS- does debug mode activate there now? If so, then it's something in your Users\[username]\Documents\EA Games\The Sims 2 folder that is deactivating the testing cheats. If not then there's something afoot in your gamedata files.

Before I do this, I just want to say that I haven't put in any mods that could change something in testingcheats recently, which makes the issue even weirder. Aside from CC, I only have custom music in my gamedata files.
I'll try this when I get the chance and notify you on whether it worked or not.

Okay, I know this thread is like months old but I finally got around to doing this:
I renamed my userdata folder and then let the game generate a new one. Surely enough, when I checked in the game testingcheats was indeed enabled just like it should be.
But what's preventing it from booting up in testingcheats, then?
Believe it or not I don't have that many mods. Only the Sim Blender, Sim transformer, the FFS debugger, Phaenoh's phonebook, Monique's computer (TakaMQ's laptop form) and Nyami's Vampire Suck Blood interaction, plus just an override made by Skell to enable the EAxis custom Change Appearance dyed hairs.
All of them don't even change anything with testingcheats.

Next thing to do would be to determine whether it is something in your Downloads folder causing the issue, or something in one of the other folders (most likely the config folder?). So backup your neighbourhoods then test your game using the new set of userdata with your old Downloads folder, and then the old set of userdata with the Downloads folder removed. Once you know which folder the issue is in, you can then work to figure out which file!

Hmm, I'll try that and provide an update.
If it's something in my CC, that would be really weird. That shouldnt be happening.

I don't need to throw all of my downloads folder in all at once, right? I just did and I'm wondering how the game will handle all the sudden burden, I'm not kidding.

Edit:

@simsample

So far, I've tried the new userdata folder with my CC:

• Dropped my downloads in
• Testingcheats worked successfully without me manually enabling it
• allmenus also worked

I've now thought about removing my Downloads would mean I'd have broken skintones & custom eyes and flashing blue objects after that.
The flashing objects wouldn't be a problem but broken skintones and eyes on existing Sims, even after putting them back wouldn't revert the Sims back, right?
I had moved my Downloads folder out but put it back in before trying without even it being named "The Sims 2", so I'm assuming nothing happened?

Yes exactly, this is why I mentioned to backup your files first. Always always make a safe copy of your game before you mess with anything, then once you've found the issue, you can restore the safe copy with no ill effects.

CAS items tend to not revert back to CC versions. They will usually revert back for template sims in CAS and Bodyshop, though (it's saved somewhat differently). Hairs/clothes will be replaced with others/EAxis, hairs occasionally show bald.

Removed/edited accessories (especially if cache files aren't removed) will sometimes show up as "white-mesh-with-black/red writing".

Flashing objects mean you don't have the master meshes. Those should correct themselves once you've got the proper master meshes in.

When you're doing 50/50, or for any other reason remove your CC temporarily and then run the game, make sure to first do a backup of your game (neighborhoods), so you don't end up with broken genetics. If you want to do a full backup for later use, backup everything including downloads (otherwise you don't have a clue what you used and how your game was set up).

I really feel like it doesn't have to do with CC, I'm not saying this just because I'm unsure with taking out my downloads, but because even in the test game with all my CC in, testingcheats was enabled plus there isn't anything there which could affect testingcheats and also I didn't put any CC in during the time which I first encountered this issue.
It must be something in config as all the enabled/disabled cheats are stored there, right?

I went to my Config folder. Guess what? A userStartup.cheat file. I changed the file extension to txt and surely enough, boolprop testingcheatsenabled false was in it. What?! Who?!
I most definitely didn't put that in. Apparently this file was made in 30th of March this year. I don't remember even doing this change. What the actual plumbob is going on.
Although it wouldn't even explain the fact that it was done before February. I'm seriously amused.
My new userdata folder doesn't have the userStartup.cheat file though, I'm literally so confused.

The only mod I know that adds Testingcheats to the UserStartup is the "SeeThem2" mod, but I'm not sure if it adds the "false" version of the cheat, or if it just removes the cheat entirely if that cheat is X-ed out.

I don't even have that mod. I didn't even know it existed until you told me and I searched it up.
I hadn't even downloaded any mods I can remember at that time, let alone install anything, which makes this even weirder.
The mods I listed above are the only ones that are in my game, apart from the usual CC (Objects, hair, clothing, skintones, eyes etc.)
Why do all the weird game problems have to happen to me?

Some versions of SimPE can start the game in debug (or not) too:

I'll have to see if I've randomly disabled this, although I really doubt it.
Do you have any other idea of what can create a userstartup cheat without user intervention?

I'd like to contribute with one additional place where one could look for the cheats:

There's also a globalStartup.cheat file located in [installation]\[newest EP]\TSData\Res\Config. It acts the same way as the userStartup.cheat, but the cheats from the global file get added to the game silently at startup. Adding this in just in case someone's searching for their TCE false line and can't find it anywhere else.

In case you have many backups, OP, it could be that one of them had a special Config folder you set up many years ago and were trying to test something out. I usually back up my folders by renaming the entire The Sims 2 folder (i.e. to The Sims 2 -backupMonday). Then I create a new folder named The Sims 2 and copy the contents from The Sims 2 -backupMonday back to it.

Sadly I don't think so. I haven't set up any config folders in the past, so I don't think that could have been the case. It's weird, though. The userstartup cheat file that I didn't remember making at all has only the testingcheats false cheat. I'm really confused about this.