Steps to reproduce (a video is attached):

1. Equip an elytra.
2. Fly, using the elytra, into a large body of water.
3. Now release the movement keys.
4. The player stays in a mix of the flying and swimming states even while sinking.
5. Press A, S or D.
6. The player model spins around.

This erroneous state stays active until the player touches the surface, even making it possible to use rockets for boost, which is not possible in normal swimming.

Edit:

1. The elytra also takes durability damage constantly in this state, even while sinking with no input.
2. In 25w05a it is even possible to touch the surface/the ground without exiting this state (you can simply float on the surface). This also leads to the player being able to swim on his back.