Under the normal circumstances actors/followers will not ride horses because they don't own them, it could be the reason this bug hasn't been noticed during 1.6 beta. Me working on Convenient Horses noticed this problem and it affects my mod by a large margin. Every time a follower gets on the horse player's mounted combat camera will return back to normal and crosshair will end up low. Sheathing and drawing the weapon will fix it. It become very annoying especially when using multiple followers. I think it is a bug with animation behaviors missing proper conditioning, because both NPCs and player use same behavior set. If Bethesda read this then I'm happy and I hope they address this in the future but if they don't is there anyone here with the knowledge of altering these behaviors by adding conditions so that camera Z doesn't change if horse is mounted by a NPC?