A quiz with a maximum number of attempts can't be attempted again if the maximum has been reached and an attempt prior to the last one has been deleted.

If you reach the maximum number of attempts in a quiz with limited attempts and then delete an attempt prior to the last one, there still won't be any attempts left for the user.

Moodle seems like it's validating if all the attempts have been done only by verifying the number associated with the last attempt, even though previous attempts have been deleted.