@dave Do you have any collection of potential issues with persistent grades? I looked for them, but found nothing.
Some days ago, we bumped into an issue related to persistent grades, and/or probably visual blocks. The issue may be caused by a race condition or similar, though we were not able to reproduce it since then.
The progress page resulted in an error when persistent grades flag was enabled. Enabling persistent grades flag seems to have broken the cache (or visual blocks?) somehow and resulted in a KeyError in models.py:
lms/djangoapps/grades/models.py", line 597, in read
prefetched_grades = get_cache(cls._CACHE_NAMESPACE)[cls._cache_key(course_id)]
Although this issue happened, did not affect all users just some of them.
Have you ever faced with similar issues?