If you try to split a multi slot when you're over capacity, it won't work.
When you try this near the capacity limit, it won't work either.
- I think it's trying to create a duplicate stack first to split that, which sort off double the volume of that stack (and that makes it seem to go over capacity)
How to reproduce:
# place a multi slot stack in an empty inventory.
# increase the multi slots volume by adding blocks, when it takes in 49.9%, try to split it
# it works
# do the same but try it when you're over 50% capacity used
# it won't work
This makes me conclude that it indeed creates a double multi slot stack somewhere and it uses the volume of both combined to check.