When I am in a doorway and open it I go into swim mode (at least I think it is). This has happened several times. It was never noticed in a previous version. I attached two screenshots one where I am horizontal and one where I am vertical. They were created during the same incident. I hope this helps you.
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Seems to be because, in the latest snapshot (19W14A), you use the swimming animation when you are in a half-block gap. Presumably, this would also happen when you are competely incased in a block.
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Also happens when you drop sand on yourself, so my theory is probably correct.