It happened on my site too. I think the email doesn’t work when the post wasn’t added to the review queue because of a flag, but because it requires aporoval.
I first noticed when I was testing the Approve unless staged site setting and deleted the user when the post arrived and required approval. This was even more confusing as the user doesn’t really have an account. They only used email in.
I would be grateful if, in addition to the fix for non-flag reviewables such as post approval, a different handling method could also be found for staged users, who do not actually have an account.
(Similarly, I would still be grateful if the email were only sent once deletion was successful, if custom flag reasons worked, and if forums where the guidelines are not public did not refer to them in the email)