I use the mod ReStack (https://mods.factorio.com/mod/ReStack) to change the stack sizes of ores to 100, but after I installed this mod, the stack size of coal (only coal) changed to 200!

Thanks.

Hi,
My mod does not change stack sizes. It's on the other mod side.
Have a look here:
https://mods.factorio.com/mod/ReStack/discussion/5d5bf16084c1a7000df0c0c0
Also here:
https://mods.factorio.com/mod/ReStack/faq

Hi.

I tested here removing your mod and the stack size of coal drop to 100. When I added it again, the stack size of coal increase to 200.
Of course it can be a problem in another mod, but I didn't do any more tests. This behavior is not so bad for me to worry too much!

Thanks.

The issue occurs when both mods are present but this is something I can't help with. Re-Stack is the one that would create the problems. As per Re-Stack FAQ the owner of Re-Stack wont fix issues like these neither.