I'll just make some definitions here so I don't have to repeat myself
Candidates: the characters in the bottom row, available for sacrifice
Death row: the row with Victims
Victims: Characters in the Death row, selected for sacrifice

Right then. In the sacrifice screen, the Death row contained more boxes than the usual ten, they continued off-screen.
Click a candidate -> it is dimmed as usual but appears as two (cloned) Victims.
Click a dimmed Candidate -> both Victims disappear
Click a Victim -> only one of the clones disappears but the Candidate is un-dimmed.
A maximum of five Candidates can be selected.

By repeatedly selecting Candidate - Victim - Candidate - Victim, etc. I now had a death row containing nine shaman Eddies and a cultist warden, but the only Candidate dimmed was the Warden. The sacrifice counter was as high as expected for these Victims.
I performed the sacrifice (for science, of course!) and got a quite underwhelming eleven rare souls (as expected for a shaman and a warden, I suppose) and lost not only the warden but also the shaman (who was not a selected candidate).

I'm Olf-5477 in case it helps with finding the error
If possible, I'd rather have the shaman back than keep the souls