When I right click on a 50000 stack and put it in a box, it splits it into 10000. This is how it should be
But if I put a 50000 stack in the backpack, it stays that size, and I don't want that.
I use
StackSizeController and ChestStackMultiplier
What do I have to do to make this work for the backpack?

Try latest version

Did not help.