Summary:
Action bars reappear when changing dimensions and re-entering the world. A complete restart of the game solves the problem. Does not affect 1.21.0!

Steps to reproduce:

1. Place boat/summon camel.
2. Sit on the summoned entity.
3. Dismount from the summoned entity.
4. Teleport to the nether/end by any means (or re-enter the world).

or

1. Run in any world:

   /title @a actionbar Hello World

2. Exit and reenter the world

Observed result:
The action bar text lingers between dimensions and worlds. Restarting the game fixes the issue.

Expected result:
No action bar appears.