Walk away out of visual range and come back. It's a network issue thing (server fails to tell your client that you can see them).
Yea been doing this, is there a proper fix for it?
It just happens based on server load I think. From a technical perspective it could be some strange glitch where somebody talks to an NPC and causes some blocking I/O right when the server is supposed to tell your client to display it, but who knows. I think it happens on all platforms even.