Facing Vas’no boss in Dalaran Heist. I had five HP remaining and Vas’no went face with Lava Burst.
After the first cast, Wondrous Wisdomball did the Ice Block/Immune effect, but the second casting of Lava Burst from Vas’no’s passive hero power still killed me.
To be clear, you took no damage from the first casting, but full damage from subsequent ones?
Had a theory that the “prevent damage” part worked but the immunity was late in taking effect last time this was reported, but OP wasn’t too clear about exactly what transpired:
Presumably doesn’t affect the normal Ice Block because of a bug(?) which persists the “prevent lethal damage” effect until the end of the turn:
https://github.com/HearthSim/hs-bugs/issues/1175