Seraph of the Suns says, "When this creature would take damage, your planeswalker takes 6 damage instead."

This creature was cast during the Temmet fight in the new Trial event. I went at him with ObNix. I dropped a Meteorite which tried to do damage, and it was prevented (as expected), but when the Seraph's effect went to damage Temmet instead per the card's instructions it did zero instead of six. A couple turns later I used Ob's second ability (Torment) and instead of destroying him, a red "-0" pops up as if I had instead attempted to do damage to him.

So...is this creature just flat out unkillable right now? Was this particular one just bugged? I get the bit about him not taking damage, but he doesn't seem to be damaging the summoning planeswalker as expected and non-damaging destroy effects don't have any effect on him either.