This has happened to me twice, a Reaper vanishes while it’s icon still appears on the map. You can stand in the exact coordinates of the icon and there is nothing there. If I quit the game and wait an hour or two, when I return to the game, the Reaper is back and visible.
The first time it happened the Reaper Icon was at -1225.414 -1548.444 just north of Anl 182 Hasslehed. The icon showed on the map, but standing on the exact location one could not see any Reaper.
The second time this happened (different Reaper) was at -2621.916 1740.259 just north east of Anl 144 Vidsele.
In both cases the Reaper returned to the surface and could be seen interacted with after quitting the game and then restarting the game. It both cases I was kiting the Reapers when they vanished. These are the only two incidents out of at least a hundred Reaper kills, so it does not happen very often. These two were about 50 kills apart. I am playing via GeForceNow, so the machine is pretty robust.
Thanks for such a great game, even with the odd bug like this, it is outstanding.