You must be in VR, without FBT.
  1. Put your avatar in temporary pose space. Easiest way is simply use a GoGoLoco pose OR Set Horizon Adjust (unsure if this uses Pose Space but is also affected).
  2. Move in your playspace (for best results, move vertically), weather it's physically or using OVRAS.
  3. There are two methods to cause this issue: Have another player switch avatars. This avatar must be newly loaded into, where it has the "Unpacking..." loading status for you, OR
  4. As another user has mentioned, have another player adjust their avatar height.
Note: For whatever reason, avatars recently loaded into already, avatars loaded from players that just joining the world, or hiding and showing them, does not reproduce this issue. It's only avatars that have been switched to that needs to unpack before loading.
Result: The Temporary Pose Space reactivates, essentially resetting your viewpoint.