I had this issue yesterday, was talking with my partner and and I noticed in background that screen turned black, so I went to check and it was said that "You are dead" , at first partner said I got shot...I was skeptical about it, so I said "no", cause there were no bullet hit nor that someone actually shot me cause no sound of it, but later on after some minutes 3 or 4 I heard gunshots, I informed my buddy cause he was coming to search for my corpse for loot, but it wasn't there, enemy must have hidden it or it vanished when I went out from server, cause respawn did not work.

This also happens when you alt-tab out of the game.

I accidentally alt-tabbed out instead of pressing shift-tab to test it out and when I alt-tabbed back into the game (instantly, tabbed back) I was dead.