This seems to have begun with the new extended clipping range for players. When a player enters your clipping range for the first time while riding a horse they appear for a second standing on the horse then the horse continues on past you without the rider and they remain floating in the air. I am able to target them as they float but in reality they are on the horse. Even so, any attempts to cast a spell on them gives a message that they are too far away. When they leave your clipping range they reappear just as they are leaving.

Screenshots show what I am describing.