When initially entering the world, gamma is as vanilla even if tweakGammaOverride is true. It takes effect only when disabled and then re-enabled.

Same. I've noticed this to as well.

I also started experiencing this, but only when I just recently upgraded to 1.19.2 (tweakeroo v0.14.3, malilib v0.13.0).

It's a known bug with the way the mod stores and/or restores (or doesn't) the gamma value after game start. It just doesn't apply properly when the game starts.

Ok, well, the bug didn't start showing up for me until this most recent update. Is there an official bug this should be linked to?

I think it was always broken/not working properly, but the 1.19 community version had a workaround/hack fix for it, but my official 1.19+ versions again don't have that fix, as I want to add a better fix for later on.

Ooh. Yeah, that's likely.

Would it be more worth it just disabling the file that resets the gamma level? is that possible or is there more then that?

Yup, I can confirm that the gamma override needs no be reenabled every time when entering a world or server

I forked the mod and fixed this issue: https://github.com/DarkCows/tweakfork-fork
Whenever the world loads I check if the gamma override setting is true and change the MC gamma setting.

Same.
Why is #434 closed?

Goes still for latest stable version 1.20.4-0.19.1.

This is intentional as far as vanilla minecraft. The fix of MC-51418 prevented values outside of the normal slider ranges to be saved by the client. There's a mod by apple502j that "unpatches" this called FiveOneFourOneEight, but it hasn't been updated to accept newer versions in fabric.mod.json yet.

Tweakeroo does not rely on the vanilla value being saved, it's supposed to always re-apply the value after the game launches, but that code has just been buggy/broken/using bad logic for ages. The fix has just kinda been waiting for my mod rewrites... (although the fix isn't large or difficult by any means ofc, I just don't put any effort to the old code mod versions anymore...)