VRChat Version: On the IK-beta build and on the live build 2022.1.1p5, build 1174.
Description of the bug: When using various seat prefabs in different worlds, avatars typically lose all tracking, sometimes also no longer tracking where one's playspace is. With AV3 Default, this breaks if you wait until the avatar fully sits, whereas with custom animations with AV3, this only happens if you move during the delay before the sitting animation occurs when triggering a seat.
Steps to reproduce (how did you encounter the bug?):
These two videos display a variety of scenarios of difference sized avatars both default AV3(Tireur) and custom AV3(Raven).
https://discordapp.com/channels/189511567539306508/946881839400882196/963937487556862062 - This begins the discussion on this issue as well in the beta channel.
Since the video is a bit unclear:
If default AV3 avatar:
  1. Sit down in a toggleable seat.
  2. Wait until the avatar has fully sat down.
  3. Move the joystick to get out of the seat, and tracking will be broken.
If custom AV3 avatar:
  1. Sit down in a toggleable seat.
  2. Immediately move the joystick before the avatar gets properly sat down in the seat.
To fix the tracking in both cases, you do the opposite. Move immediately after sitting for default, wait until fully sat down for custom.
Any extra files? (output logs, screenshots):
https://drive.google.com/file/d/1o4BhSEvHCE_WfhsensPn1n1MfCzhMy-l/view?usp=sharing - Snippets of the output log during the time I was testing.