Riding entities whose hitbox does not intersect the mount's chunk get dismounted upon relogging

*Partner Rank:*

3

*Partner Team Reporting:*

Everbloom Studios

*Verification builds:*

beta 1.11.301.0 (Release_2199935) and beta 1.12.0.3 (Release_2120526) on Windows 10

*Summary:*

If entity A is riding entity B, and entity A's hitbox doesn't intersect the chunk in which entity B exists, then entity A will be dismounted if the player relogs.

*Impact:*

This bug keeps us from creating interesting multi-part entities using rider seats.

*Repro Steps:*

1. Add the provided behavior pack to the world you're going to test this in.
2. Join the world, place down a boat with ample space in front and behind the boat.
3. Spawn two mobs and push them into the boat. You might want to put two different mobs in to see which one dismounts.
   Once you push the second one in, the mobs will appear 8 and 16 blocks away from the boat, respectively.
4. Close the world and open it again.

*Observed Results:*

Either one or two of the mobs will have dismounted the boat, depending on the position of the boat relative to chunk borders.

This behavior can happen naturally, and this can technically be reproduced in survival with no behavior packs, by carefully positioning a boat on a chunk border and pushing small mobs into it (like rabbits). We don't happen to notice this bug though because the "pulls_in_entities" property in the minecraft:rideable component makes the dismounted mob get back into the boat. 

*Expected Results:*

Neither of the mobs should have left the boat.

*Screenshots/Videos attached: Yes/No*

Yes

*Regression Builds:*

Unknown

*Notes:*

Video link