I found another instance of it happening, this time in standard arena.
In this instance, the defender was using only Leorius, so it is as pure a setting as I can imagine for testing. Leorius used Awesome Roar on his first turn, and it killed Siegfrund. There were no other champions on their team, so it was impossible for Siegfrund's passive to be on cooldown.
It seems that the passive, in this situation, is being blocked by True Fear, perhaps with the stated 50% probability of fear effects. If this is intended, it may be appropriate to have the in-game documentation reflect that True Fear can also prevent passive skills from activating, including passive damage blocking, shielding, and healing skills.
I was not expecting this to happen, so I did record anything, but the test should be easy enough to replicate.