I explained it badly: the post is indeed deleted, it’s just that the page isn’t updated in real-time to reflect that the deleted user’s post has been deleted as well when we deleted the user.
More an UX thing. There are several other things like this where the page isn’t properly updated after an action.
Or, more similar:
If we are in an admin view of a user, and we can’t delete the user because he has posts, if we click on the “Delete all pots” button, the button to delete the user won’t appear until we force a page reload.
But that’s a very tiny issue and I don’t see it being addressed.
However, the issue which my present topic is about is slightly more annoying when we review a lot of users/posts in a row (every day!) because of the error message.