When the character is Sapped, it randomly moves, or let's say better: it rotates a little bit in some direction, when it should be stuck in place. I think it's something related to the wrobot "unstuck", in fact if you have "Dismount when stuck" feature checked, it will dismount if you are sapped. So in few words: the robot thinks Sap is a stuck situation, and forces some movements in order to unstuck (at least this is what it seems to me). It should just stop everything unless in the class fight there is something that can remove sap (medallion, bubble, and so on).
This is a small bug of course, but I'm testing some profiles in Battlegrounds and I see this happening a lot, it looks very weird, and may be even being classified as "hack" if reported with video proof (kinda like the hack that allows you to move while cycloned, or rooted, same concept I think).
Thanks for reading