Despite this being a pretty well-known thing, nothing relevant appeared upon searching, so hopefully this isn't a duplicate.
Fence gates, pumpkins and jack o lanterns (but not melons) require certain blocks underneath them to be placed, but will remain if said block is removed. This bug was fixed on the Java edition fairly recently (MC-1947), so it makes sense this should follow suit.
Interestingly, the blocks considered acceptable vary between the block; fence gates can be placed on leaves, whereas pumpkins and jack o lanterns cannot.