When trying to evenly split a stack with the middle mouse button (mouse wheel) it is trying to place a whole stack into the slots instead of a splitted stack.
Also, the "remaining items" number at the orginal stack is reduced by 64 per slot, resulting in red negative numbers.

This occurs with every stacksize >= 2.

A video showing this bug:
https://www.youtube.com/watch?v=6KVN0GwGDCI

I also attached a screenshot.