Decorated pots that can be crafted using pottery sherds (which have uncommon rarity) won't inherit said sherd rarity. Same would probably apply to decorated pots found in trial chambers.

How to reproduce

1. Get 4 of any sherd.
2. Craft them into a decorated pot in a crafting table.
3. Notice how the decorated pot has common rarity.

Expected result
The crafted decorated pot would have uncommon rarity.

Observed result
The resulting item has common rarity instead.