its in the code. spells.cpp
Mob::CheckStackConflict
Mob::CanBuffStack
it is known to be incorrect. but its going to take a fair ammount of work to fix it up in all cases. Please fix this if you can, it would be great to have working correctly
I highly recommend using lucy as a source for stacking information, they seem to have it right. Its possible that an entire rewrite of the emu spell system might be needed if you can have only part of one spell overwritten by another spell. Keep in mind that there is rules based on the effect in each slot, as well as explicit rules in some slots (cant recall specific example)