Have noticed it in both but more in operations and there are instants based
I dont do many heroics, but in OPS and FPS yes
You cannot click them in the OPS/Group frames until the /follow you then the show up and become visible (most times in the wrong location and you cannot revive as it say "out of range"
The Dead person does the /follow and is visible again and 99% of the time in a different location
Hope this helps