You're 100% sure it was the HP burn that killed the spider, and not Geomancer's passive?

I don't think that's possible. If there's no HP burn active, no damage is reflected. If damage is reflected, then there necessarily must be an HP burn active since the debuff is triggered and decremented prior to the attack.

Yah nvm, just badly worded on the skill description. Thought the 30% version didn't require HP burn.

Still - HP burn ticks *before* the attack, so is it possible that maybe Geomancer passive still activates if that last tick removes the HP burn?

I'm using Geomancer and Ninja for CB, so I've had plenty of opportunity to watch the interaction. From my experience, I've learned to use Ninja's A2 whenever I see a HP burn with a duration of 1 (or when the next attack is the stun), because the next time the boss takes a turn, the burn seems to expire before the attack as no damage gets reflected.

With that being said, the HP burn debuff does 3 things when it ticks: applies damage, decrements the timer, and expires if applicable. It's possible that it's programmed to tick and expire first, and then apply the damage. In that case, the spider would receive the damage only after the burn is removed, and thus technically die without the HP burn active. Since I can't readily think of many situations where the order would matter except this one, I imagine it would be an easy thing to overlook. 

I didn't pay close enough attention if Geo's passive killed the spider or the HP burn.  Either way if there was an HP burn on for 1 turn, spider dies, then he should stay dead.  He shouldn't revive just because there's not an HP burn on for a future turn.  He should be dead! 

I have since played again and the HP burn had 2 turns left, so Sorath stayed dead.  I still think this may be a glitch that should be fixed.  If the HP burn kills Sorath he shouldn't be able to revive. 

I've never heard of, or seen this bug, personally. If you end up doing the fight again could you record it at the end?

Doesn't really sound like a bug to me.  HP burn is active as soon as it is applied, so you could kill it before it ever activates and revive would be prevented.  If you then let the spider take 2 turns and it's still active that's like 3 turns of active HP burn (not damage, but for prevent revive purposes yes). All of your champs would have 3 turns to kill it, no?  Another way to look at is that the HP Burn applied twice, and now you want it to stick around a tiny bit longer?

"From my experience, I've learned to use Ninja's A2 whenever I see a HP burn with a duration of 1"

This is true, it expires thus it counts as it was killed w/o HP burn being applied. It is kind of misleading though, as you'd expect that the burn would be active for 1 more turn, but instead it ends in same turn if it makes sense.