hi my user name is meica and the bugs with the multi player mounts are happening on all my characters regardless of lvl. i cant say what quest i was on as i was helping other players. the mounts including temp ones glitch out when you take another passenger into buildings, you find your passenger gets stuck to you, this happens in all realms and all areas as a friend and i put it to a test for recreation purposes, unless you move right away from your passenger they will stay stuck, it also happens whilst on sigils too. i also tried this out with other people to make sure it wasn't just my friends.
DOUBLEc8527 reported this issue back on april 22 2021 and has a couple of videos to show what it does:
This happens to me as well I think it is a little annoying but for now just make the best of it! For instance with this glitch if your friend is gone and you are farming gold (I did this with my friend) then you can just put them in your mount and keep on farming because they follow you! But ye I want this fixed as well but for now just use it for the better!