OK so I'm not confident I can clearly put that into words but I'll try it.

Say I have a stack of 10 potions in my inventory - 10 being the maximum size of a stack. I have chest with a stack of 8 similar potions. I split the stack in my inventory to take 4 potions out and only keep 6 on me, I drop the 4 potions on the existing 8-potion stack. The stack in the chest is now maxed out with 10 potions, but when I try to create a new stack with what should be the remaining 2 potions it actually still take 4 potions out of my inventory, and now I only have 4 left.

In other words, when you partially unload a split stack, it does not reduce the size of the split stack.

I can't for the life of me remember if it always worked like that, but it's freaking infuriating. I try to get rid of 4 potions, and keep my chests a bit organized and now I have to juggle these stacks to make sure I don't store too much.