I wanted to take out some health potions in Auridon, but there was no "split Stack" option. I had to take out the whole stack, split it in the back pack and re-bank the rest of the stack. I checked out different stackables in Auridon, Stormhaven, Daggerfall and Eastmarch. None of them gave me the option to split the stack in the bank and only take out the amount I wanted.
This was working fine earlier in the week. In case it matters, I'm on PC EU