I noticed that my 12 citations entered in the post body was displaying as 11 citations in the list. And instead of simply having deleted a single problem citation, the stack was shifted up, so that the citation linked number referenced a citation intended for the next cite location in the body.
Seems I pasted a doi into my WP editor that contained some kind of non-printing character in the middle of it that was not obvious in the WP editor. When I saw and deleted that character in an external editor and pasted back all worked fine.
The expected behavior would be an error message or at least a missing cite number, and certainly an otherwise accurate remaining stack.