Removing the saddle a player is using whilst on a horse will teleport the player (in my case) from coordinates (-217,  111, -5109) to (-201, 107, -5115). Hasn't happened since and there doesn't seem to be a straightforward way to reproduce the bug. Note that the bug occurred whilst on a realm (no one else was in the realm), game mode survival. I do not own the realm so I am not sure on seed and difficulty. The attached video demonstrates the bug (captured using xbox game bar recording the last 30 seconds).