I am playing on a mod pack assembled by a friend of mine, and when exploring the Nether one of my friends complained of suddenly getting instantly killed by nothing.
Upon investigating myself, I found the sulfuric vents, when going near them, unusually rapidly damages the player, with no delay between damage ticks whatsoever, resulting in the player's death in at most 20 ticks (one whole second). I believe this must be a bug, or if this is intended, a severe design flaw.

I could not find a similar issue listed here, nor any postings of incompatibilities elsewhere.
A list of mods in the pack is attached.
Mods.txt

My friend has finally identified that the culprit seems to be "Iron's RPG Tweaks", a module in that mod apparently changes how damage invulnerability works. If anyone else is searching for a solution until there is better compatibility- Disable the "Combat" module in the config.