There was a bug here; it was processing things that were "flagged for moderator attention" (i.e. bespoke messages), but it was not processing the spam/offensive flags. This is because back in the depths of time (before we explicitly tracked flag outcomes), such votes on deleted posts were ignored in the moderator-facing UIs. It looks this didn't get updated to explicitly process the outcomes when the community votes to delete a post.
This has been fixed in the code (and will be deployed next release, usually within a day), and I will try to apply the change retrospectively.