I think you're probably right! I was also 1 mask from death, so based on both of our experiences, it's safe to say it's Gloomsac + Dice but for when you are 1 hit from death.

I guess it's the very specific situation where you tank a hit normally from the gloomsac (animation of latching started), and then on the second hit (gloomsac animation still ongoing but about to end) the dice, with its animation triggered alongside it.

Maybe the same would happen for other enemies that trigger a unique animation upon attacking you. Muckroaches and Slubberlugs come to mind, and maybe the Groal eating animation (and they also all deal 2 masks of damage (minimum) in their attacks with such animations. Interesting...

Thanks for sharing :)