Entities riding on a mount with a seat offset greater than 16 blocks are dismounted after a relog

The bug

If a custom entity has seats with a position distance greater than 16 blocks, entities can enter those seats just fine, but after you relog, they will no longer be riding the entity. Riders on closer seats are unaffected.

How to reproduce

1. Download and open the attached world
2. Spawn some more witches at the boat, making them ride it
3. Verify the witches are properly held in place above the red concrete to the left
4. Close and reopen the world

Expected results
Witches are exactly where you left them

Actual results
The newly added witches were dismounted and are wandering around

Notes
I've also observed some witches closer than 16 blocks dismounting, without particular consistency.