Ultimatum Damage Bug (probably)

Just died (nearly) instantly at starting an Ultimatum before any enemy unit spawned.

Tier 16 map, 0 enemies left, started Ultimatum with "Buffs expire faster" => Saw my life be drained from 100% to 0% in around 0.5 seconds, before any enemy unit spawned.

My build works around -200% lightning resistance, so my only idea about this happening would be something like a "lightning damage over time" (lightning hits/... are transformed) that spawns at Ultimatum start.
I could not find anything like that in any information on the internet, therefor I suspect some bug there.

If anyone can tell me if I am missing out something, and why I am stupid, I would be pleased.

Otherwise don't hesitate to sign up here if that happened to you aswell.

Thank you.