Not instances specifically but instances of it happening... I've seen it happen before on planet.
This used to be the case, but not anymore.
Now as long as no player is in range, all regeneration will be halted. This means that if you were hunting a mob, ist at 2%, you die, it stops but if any other player as much as enters the radar range of seeing it, then it starts again. and stops when they leave again, vise versa.
I've had times where I died at a fairly low hp, in a dense place and thought fuck it not gonna bother. went to eat some food and what not and upon returning an hour later, the same mob was still at the low hp as when i left it.
So in the long time agos, they regened no matter, and if it was in newer times, another player has been close enough to trigger its regeneration. And if its a fast regening mob itll be full in no time at all that way.