"Final Boss drops additional <currency>" altar reward not working in Sepulchre Map

Bug Reference number 2,269,507,987.

I was running a Sepulchre map with Searing Exarch influence.
On my way to the boss I got a "Final Boss drops 3 additional Grand Eldritch Embers" modifier.
When I killed the boss, the Grand Eldritch Embers did not drop.

I checked the boss arena sector with Alt and also checked my filter but it shows Grand Eldritch Embers just fine; actually in a map I ran after that one I had a similar altar modifier (just 2 Grands instead of 3 Grands) and it triggered just fine.

I got the altar on the way to the boss so it should have affected it.

Considerations:
Boss died in Ritual circle. I thought that maybe to the game mechanics "Final boss" will be the one to die in last Ritual iteration but no, it didn't drop the Grand Embers at any stage.
The map's boss has different models depending on fight stage and when it switched between them maybe some "final boss" logic went awry?

I'm attaching screenshots where the floor items can be seen [with ALT held] as well as my inventory.

The same issue happened yet again, in the same map, with the same type of Altar modifier "Final boss drops X additional <currency>", this time it was 4 Chaos Orbs. Seems like either this specific boss is the issue (because she changes form that resets her HP/status/ailments) or because of presence of a Ritual altar.