I just lost another sorc on HC mode due to the same thing, but not the same creature.
At this point i do think its a bug.
i was farming helltides in T3 world without issues at lvl 65, i already had a decent build and stats.
But then a “normal” creature outside the helltide event Literally one shotted me, ignoring my entire resist and armor, it was that unique elite map boss ( it looks like a giant tree or something ).
Not even vashan was one hitting me that easily, and i got in more than once to farm unique itens.
How is a special elite more far more dangerous than a unique boss ???
Aha… he got you.
My advice is, stay the hell away from him, he produces little bug things that explode when killed.
He’s killed more HC chars than any enemy.
You haven’t try to jump on a big group of spiders which will explode after you kill them and send you to meet Lilith 
Check you character. You shouldn’t die in helltide no matter what.
It was a poison tree elite outside the helltide that in theory should just apply dot dmg plus its physical dmg when i was near it
the thing is, i was killed by literally just one hit by just touching the poison.
I was in the middle of my rotation skills so im almost sure that i was negating nearly the cap of dmg reduction in general for that lvl of map plus the poison resist which was around 55%, by that i believe that the poison dmg itself should have been greatly reduced, yet nothing that applyed and i got killed.
Im pretty sure it was a bug and one that haven’t been fixed since the first season.
Rotsplinter
Characteristics: Huge Tree that inflicts poison damage and spawns poison spiders, leaving acid on the ground.
( https://www.icy-veins.com/d4/guides/all-rare-monster-locations-and-item-drops-in-diablo-4/ )
This Elite hits very hard and can surprise unwary players with the poison explosions.
Poison in general is overtuned in this game. DOTs tick far higher than they ought to and poison can stack very high and tic ridiculously fast.
Those explosions aren’t scaled like one might assume…tiny spider - little boom/big elite - big boom.
They all hit like a truck.
Sorry for your loss 