After having tried placing a flower pot in a decorated pot (with other items already in it), I could no longer move the stack of decorated pots or the stack of flower pots in my inventory. Since the flower pots were in my hotbar I tried placing one (this is kind of hard to see in the video) and although it could not be placed on a slab, the count suddenly jumped from 5 to 7 (the original amount of flower pots I had) and both type of pots in the inventory started acting normally.

I'll try to reproduce better but the steps were basically:

1. place decorated pot
2. try to place flower pot on top of decorated pot

Observed results:

Stack count being wrong until I place an item + not being able to move decorated pots or flower pots in inventory (see video),

Expected results:

I'm not sure. I was originally expecting the flower pots to go on top of the decorated pots as have been the case up to 1.20.50 but this probably requires crouching now.

Edit: You can see the glitch in the video but I could not find a way to reproduce when trying again. Sorry.