When you are riding and get engulfed, you should be “ripped from your mount”, because currently this behaves weirdly - you end up still riding your mount inside the engulfer, your steed is not adversely affected, and if you dismount you “float down” and the steed magically pops outside the engulfer. The size of your steed also doesn’t matter, so it also doesn’t make much sense if it’s something huge like a dragon.