When a player in Creative mode empties a flower pot, the contents of the flower pot (i.e., flower, mushroom, etc) are put into the player's currently selected slot. This is inconsistent with how other blocks and item frames work in Creative mode.
discovered while testing
MC-170833Crimson Fungi and Warped Fungi cannot be placed in flower pots
MC-169496Inconsistent behaviour when receiving items from placed blocks via right-click