This has happened twice this season haha… I’m talking to Urshi leveling my gems when all of a sudden a random mob will come n kill me. I was under the impression all random mobs dissappear when the GR boss appears.
Unless the GR boss has spawned some mobs and I haven’t cleared them all but I talk to Urshi after I’ve picked up all my loot which takes about 5 secs and nothing seems to be attacking me then.
Either way it’s no biggy, I actually find it funny.
Glad I don’t play HC haha.
Sometimes, I encounter mob spans from gr boss while the boss was already defeated.
And yes, died from those as well (sc).
Still get all upgrade chances though.
Yes, mobs spawned by a RG do not disappear when the RG goes down. So if they were somewhere far while you were fighting the RG they can come at you later. This can easily happen for example if they were fighting with one of your pets or your follower, and then your pets disappeared while you were talking to Urshi.
There are also the rare but known bug where you get a RG mob monster that is unkillable after you after the RG dies.
I have also gone on after killing an RG and found more monsters in the rift, on the next floors of the rift, etc. I haven’t tested this in a year or so, but I had tried it several times and every time I would encounter monsters if I went exploring the rift after the RG was killed.
Bottom line, don’t assume there are no monsters left after the RG is dead in Greater Rifts.
One RG to watch out for with this is Stonesinger. He spawns turrets that can be difficult to see in certain maps and/or weather/lighting conditions, and if you don’t search for them and take them out, they’ll kill you while you sit and level up your gem.
Another thing to note if using nemesis bracers if you hit a pylon after RG appears those spaws will not die until you kill them.
I know sometimes the rift boss spawns to you if you’re too far away, maybe that’s what’s happening and the mobs he spawned are still walking towards me.
What peaked my interest is that this is like my 15th season and it’s never happened before.
As I said it’s only SC so I usually just laugh.
Some maps contain traps that spawn mobs, e.g. bats falling from sky. Traps aren’t removed when RG spawn, they remain active on the map.
If you walk near the area, it spawns 3 or more mobs a second later. You might not notice, but they might follow behind you.
There’s an area on a Keep Depths map, where fat zombies climb out of the floor, with a few switches to close the holes they come out of. There is a massive gap at the top you can’t close.
These guys will continually spawn for 30 secs or so, and don’t follow the normal “all mobs disappear when RG spawns” GR rule.
Not sure it’s a bug, as it’s just an environmental reaction. But it can be annoying in a high GR when they take a while to kill, and Urshi also spawns in that room.
No, all mobs disappear when RG appear/die… however, it is possible for new mobs to spawn. From my experience, mobs appearing and not dying after the killing RG are as follows:
If you are in the Halls of Agony Map where you are in the area where the zombies crawl out from the furnace.
when there is a Grotesque mob (and the likes) that dies when RG spawns. The spawns (usually those eel/worm-like creatures) remain until you kill them.
Those Corpse Raiser that spawns Vile Bats after killing. Like #2, when they die due to RG appears, the vile bats remain until you kill them.
Those that spawned when taking a pylon with nemesis bracers on.
Prolly Nems 
Go run a GR and explore beyond the RG and the next floors and get back to me. I’m betting you will find more monsters and not from spawnables. There is a high degree of chance you will find next to nothing on the same floor the RG was killed, but you will find more monsters on lower floors. Though usually the density was lower.
I have seen this quite a few times, but its not something I have explored recently. And no I normally don’t go beyond killing the RG when running any rifts, but I had explored just to see what else would happen, etc. And always encountered some more monsters in GRs. I don’t recall ever NOT finding more.
I’ve seen this a couple of times too, though it was a fair while ago.
Thought I was onto something, thinking I could continue to kill mobs for xp in a high GR after killing the boss.
It didn’t happen again when I deliberately checked for a few GR’s after, but it’s definitely happened more than once.
Must be another weird random bug, no idea how to recreate it.
Don’t forget Perendi’s rain of rocks.