If you start swimming and hit an entity for the first time while swimming you would stop swimming for a second on a client, but not on a server, desyncing the player position between the client and server.

Steps to reproduce:

1. Set your gamemode to creative mode
2. Teleport to a nearest ocean monument
3. Give yourself a sword and start swimming near the edges of a monument while attacking entities.
4. Eventually you would get stuck, even though you're not touching any blocks