Player Orientation Not Following Gravity

[Deantwo]

Addendum: swapping avatars results in the swapped-to avatar having the orientation of the swapped-from avatar - either that, or it's random/just horizontal.

I have not experienced this. Are your two avatars maybe close to each other? Mine are in two different solar systems. This means that the current scene is unloaded from my client when I switch avatar.

Same planet.

Yeah then the avatars are in the same scene, so switching back and forth between them won't reload the scene.
Either switch to an avatar that is on another world, or restart your client instead.

Pro-tip: You can click to switch to another avatar, and then click to switch back to the first one while you are loading.