Google translate gives us this…
Good evening, is it normal to die after finishing a sub rift while raising your legendary gems? bug? in extreme seasonal it hurts! screen to support.
Sometimes, even after the Guardian has died, there may be enemies which will not die…
Also, there are some map areas which will cause mob spawns due to hero proximity, and they continue to spawn even after the Guardian has spawned, and will continue to do so until the event completes.
Sorry for your hardcore hero’s loss.
I hope you kept some spare gear for the next one.