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.
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qmagnet
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This also applies to dandelions and poppies.
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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