When you grow a tree, or are in the area next to one when it grows and the tree is a small five block high tree the player will be forced into a swimming position. Whereas in other versions you would simply get stuck in the leaf block.
This bug occurs in both survival and creative. The player doesn't have to be near water for the player to be forced into the swimming position (not sure if this would have anything to do with it).
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Duplicate of MC-147612

personally don't think this is a bug when elytra are eguipped you can get down to a 1 block tall height to get into small spaces to get spots you may have missed and i also believe it to be part of the new swimming mechanics as well you just stumbled upon another way to trigger it
👍🙂