When you have a stacked non-stackable Item (that means you have any non-stackable that has a stacksize of 2 or more) in your inventory and left-click it to pick it up with your mouse, you cannot put it back down into your inventory in 1 stack. It will just reduce the held itemstack by 1 and keep the rest in the picked up stage.

In versions <=1.16.5 it was possible to just place the whole "illegal" stack back into your inventory in one piece rather then unstack it 1 by 1.