How to re-create:
1) put a wooden trapdoor on the top half of a block at waist level
2) push it down, so it is perpendicular with the ground.
3) walk forward so you would be standing in the block if It were up
4) then push it up again, it makes it so you start swimming, although you are above ground.
Please do not mark unreleased versions as affected.
You don't have access to them yet.