The position of this "attached to your body" Combat UI only updates when it finds your body object (your character).
When you're mounted, it can't find it, so the UI stays in place.
And it goes off screen cause it doesn't take into account the size of your screen when you zoom in, it only cares about your body size and making sure it's not overlapping it.
In ortho cam you can't zoom in that much for all I know, so it never mattered.
Just use a custom client like literally the entire playerbase.