I've noticed this with every build I've played this league (djinns, grim pillar totem oracle, glacial bolt gemling). It's like AOE damage sometimes just randomly decides not to register at all.
Rather just farm something else than do a shitty version of abyss because some retard came up with a stupid idea that Jonathan loves.
I feel your pain. I've been clearing all the ultimatums out of my stash tab because I decided the only way I've ever getting a Mageblood this league is by having something like an Adorned drop and then selling it unid. Not sure why I do it to myself, at some rational level I know it's a <1% like every other boss rare drop but I'm sick of doing slow-ass Ritual and rarely even getting decent omens let alone one of the belts.