I have this issue atm. Watch closely you can see the tooltip flickering (like mouse over). The reason this happens is because of Crusader procs and Dispels happening around me. Reset my UI and still happens?
I do not use any tooltip addon, just anchored to mouse via Leatrix (this bug happens without any addons)
I have done a full UI reset, removed Curse and ALL addons (variable settings too) so it doesn’t recsync old addon data, reinstalled WoW completely fresh.
At this point the intro comes up, I scale my UI down and move a few Blizzard settings around so it looks somewhat playable. Next, I install Curse again and start re-downloading only specific addons such as Leatrix and Bagnon. The bug comes back (tested in AV).
This gets very aggressive in PvP/Raid, it happens when a warrior/rog procs Crusader or a Priest uses Dispel on someone (from my experience of having this bug for a long time).