When you ride a entity and apply a motion to it, and teleport it causes a desink which can get you stuck in unloading chunks.

How to replicate this issue:
Make a flat world and follow these steps.

1. Place 2 repeating command blocks.
2. Command 1:

execute as @e[type=sheep] at @s run data merge entity @s {Motion:[1d,0.0d,0d]}

1.  Command 2:

execute as @e[type=sheep] at @s run tp @s ~ ~ ~

1. Spawn a sheep with spawn egg.
2. run in chat: /ride @s mount @n[type=sheep]
3. You will now ride the sheep to the edge of the world, the new chunks wont load, you get stuck in unloaded chunks and you cant dismount.
4. To release yourself run in chat:

   /ride @s dismount