Very strange. If you place a flower pot down, then place a mushroom in the pot, the mushroom will not appear unless you create a block update. I haven't been able to figure how close the update has to be. Testing has mushroom appearing at different block distances based on coordinates it seems.
Linked issues
Comments 3
Duplicate of MC-29505, please use the search function to see if your bug has already been submitted. Currently over 57% of tickets are being resolved as duplicate.
This is also the case with Cacti and Dead Bushes and some Flowers