I've had this happen to me on the moon. Entering the Hellmouth from Anchor of Light, I skipped the loop & jumped up on the rock & dropped over the top into the hellmouth area, only it didn't fully load. I got caught under some geometry, but was finally able to climb back up & out into the path linking the two.

Huh... I've never consciously noticed it, but it does appear that there's a grenade indicator when you get close to the mines. If you watch the first mine (the one you run past), a grenade indicator appears to your left as you run past it, then moves behind you (in the direction of the mine). Then a new one pops up on the mine directly in front of you. Which all means that there probably was not an ACTUAL grenade present which went off and killed you.

Also, your cause of death is listed as "misadventure," which is usually reserved for smashing yourself on a wall or falling down a hole.

And also also, that last mine wasn't active nearly long enough to have gone off, PLUS a damage number popped up when you hit it. You definitely cleared that mine. And somehow died from the impact. The only thing I can figure is that the forward movement from your melee somehow gave you enough momentum to smash into the mine with deadly force.

Yeah, I just saw the amount of damage it did to me... 933, holy shit I must have hit the mine with my face at mach speed!

You can get Orbs out of mines if you have a "generate Orbs by killing Fallen with a melee" gloves, no matter which "cell" you're in.