Check your auto graded assignment settings. Under the Review options heading, in the Later, while the auto graded assignment is still open and/or the After the auto graded assignment is closed columns, you probably have Scores unchecked. Each time a student completes a quiz, these settings are consulted and the scores will be hidden from all students. If your students do not all take the auto graded assignment at the same time, it can look like auto graded assignment scores reset themselves to hidden randomly, even after you unhide them.