@amanda_greco instead of using Hide Grades, try using a manual grade posting policy. When I tested this just now, the new quiz submission score (after essay grade entry) stayed hidden. In the new gradebook, Hide Grades is literally that - for grades which had been visible to students, Hiding withdraws them from student visibility. (Hide is not Mute - something we're finding needs clarification with teachers used to old Mute.)
Regarding:
Is this a glitch or a problem others are seeing?
It's probably worth reporting to your support team. I think it may be that scoring the essay question updates the grade; this "fresh" grade is then immediately released under what may have been a default automatic posting policy.
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