Grave Trough - Ritual Vessels

Heylo

I killed the Grave Trough boss within the ritual area as shown, and completed the ritual afterwards, but when I used the vessel the boss is not included. Apologies if there is a reason for this, but it seems bugged.

My only thought was that because you activate the sarcophagus to get in, that somehow makes the boss not be detected as within the ritual area within the arena?

Thanks!

PS Beautiful map and boss, please pass my regards to the designers on that one :)

PPS Can we have a screenshot upload function? I really don't like that I am required to use an external site to report bugs (potential bugs at least!), that is wrong.