HP Overlay sometimes displays HP info from two separate NPCs
Sometimes, after either combatant has fled from a battle or you have killed an NPC, a second HP overlay bar will display during/when engaging combat with a new NPC (of the same type?). The HP bars will swap intermittently between themselves which causes a flickering effect. This typically clears up once combat has ended again, though it can sometimes persist for a few battles, which can cause even more HP overlays to display (as seen below).

Honestly not even sure why this would be possible. Is there some loop on purpose for drawing additional bars? Was this an original design feature? Probably not since it flickers every frame. Will need to introduce an "active npc" concept to the client. Alternatively could let the user tag as many npcs as they want manually, but, seems like a lot of work for no reason.
I think could be a failure to detect the active npc they are fighting against. @nickzuber had tried to get the active npc but is not perfect. He hadn't meant to have shown nearby ones, just probably in case like the image, where the first mobs bar was likely not the correct one.