I know, this happen still again. Followers fall trough mesh, on restart, or when you leave renderdistance and come back.
They can even go far down near the bottom of the map. Can also happen when player dies.
I tested it with several saves, without any mods, and it can still happen.
Placing them on foundations can help, but not fully. Only exeption i found are treeplateforms.
This has been reported several times now. There different posts trough the forum.
