if you put gravel where it normally will fall down it wil disappear, if you then place gravel on the same place it will float
NOTE: This only happens when you have a clock with /kill @e[type=!Player]
Linked issues
duplicates 1
Comments 2
Dupe of MC-56037
When you place the first gravel, the falling block entity it tries to produce gets killed before it can delete the first gravel. making the block disappear client side, and stay server side
if you place any block where the gravel was, it turns into gravel.
posibly relates to MC-54026
(because you are creating a ghost air block inside of a gravel block)