A student reported this problem to me.
I will commonly lock quiz grades in the grade book, but open the quiz date up in order to allow students to practice more quiz attempts prior to an exam. After the student completes a quiz attempt, Moodle shows a button to Re-Attempt. When this is clicked, the student gets a strange screen that even provides a button to "re-grade." This doesn't make any sense for a student, since they can't regrade even if they wanted to. (See graphic.)

This seems like a "bug" to me. I suggest that either this warning screen be removed, or that when the quiz is finished that somewhere, such as right above the "Re-Attempt" button, that a one line note be shown to the students, such as "This Quiz is locked. Your quiz grade will not be updated."