I'm trying this with cobblestone stack. In 1.16.4, shulker box will always reset back to 64 if I put 65th cobble inside of it. Chests works ok, albeit their counts are stuck on 64 - but when I move the stack back to player inventory, it will say 65 there. But this doesn't work for shulkers.

I'm using as simple items.yml configuration as this:

allWorlds:
  default: # Everything defined here will be applied to every player, group, inventory within every world UNLESS overridden below
    MIN: 127