I'm using a mod that set the stack size of coal to 500, and obviously I don't want to dump 500 coal in a single furnace, so I tried to set it to 5% instead of 100%, but it doesn't work.

Whenever I set change the "per player" setting, and click confirm, it instantly reverts back to the default for some reason.

EDIT : I managed to change it, but it was a massive pain in the ass that is apparently vanilla : https://www.reddit.com/r/factorio/comments/bumkti/comment/epir4c1/

LOL I just tested this and that's so strange.

I had never noticed :-P Of course the mod settings are configured by default how I want them, so I don't really change them much for myself except when testing.

Okay but like you said there's nothing I can do about this, it's just how the game behaves.

Thanks for the bug report anyway, please report again if you find anything :-)