-Tetsu-

nope, not hot and not humid there. but I've checked the only original cheki folder I have and it's not happening there, so it really has to be that material. I have some spare sheets from cheki orders that are made from the same material, cutting all of them up rn and putting it inbetween the signed cheki and the holder so I guess it should stop getting worse now.

I'm always scared to destroy them or lose them so it's the first thing I do before putting them into the folders 😅😅 😅

If you have that problem with only some of them it's gotta be the material. By "original" you mean "meant for cheki", right? Probably it's a different plastic, better prepared for those chemicals.

I have a lot to scan next time I go to my parent's place XD I used to scan them immediately too.

yes, that's what I've meant, haha. it definitely is another kind of plastic, it looks and feels completely different.
Gonna have to scan my newest chekis again too ... since I've got a new computer the scanner software fucks up the quality of the scans ugh