Holy shit, people, I understand your desire to perch yourself on the high horse of "the problem is not on our side, it was sloppy modding all along", but in fact you're just admitting you're incapable of maintaining backwards compatibility and basically ruining 15+ years of modding for the sake of claiming people did things wrong. Who cares, things worked well for fuck's sake and now they don't because it is WeiDU that changed, not TobEX or anything!
FILE_EXISTS and FILE_EXISTS_IN_GAME is like a dish washer and a washing machine.
Two different functions with two different purposes.
The previous WeiDU author/maintainer -- Giovanni/Bigiani/Rosellini/Fettucini (or whatever his name is, I forget) -- decided to conflate the two and made it such that your washing machine can not only wash your clothes, but also
accidentally wash your dishes.
If you're a programmer, you would expect a function to do exactly what it claims to do. Not additional/unwanted/unrelated/undocumented things. Because that will lead to a slippery slope and bad things will happen.
You should be angry at the BG2 Fixpack (and those other mods) instead for not updating. Not WeiDU.