I present you my wat. Everything was fine, then one step later it’s all that. Either I stepped into a fire (without warning) or I got crushed by a giant pulsating wooden heart. I, uhh, I played with fire there earlier.
seems like a fire spreaded into your tile or you’d get a warning. it would also be invisible if blocked by a wall.
or: NPC with flamethrower
Or Both, Either way sorry for your loss.
Graphical glitch, I first noticed with the strange-temple fire traps, should have guessed it would recur: Hot Air (invisible!) field blocks the Fire field from displaying. >_<
I’d say that fire instakilling a healthy survivor should also count as a bug.
Providing no Flimsy/Frail/Fragile, yeah, but full-body in raging fire isn’t a joke. (Nomex not protecting is a known issue, unfortunately. 
)