by Neil Stapleton.
We've experienced the same issue in 2.9.1
Like you we've narrowed it down to being related to specific users. I have a suspicion it could also be linked to assessments that were restored from a pervious version of Moodle (2.6), but haven't been able to prove that conclusively.
We are just about to upgrade to 3.0 and are trying to recreate the conditions to be able to test whether this will still be an issue.
I've seen a few trackers related to these filters e.g. (https://tracker.moodle.org/browse/MDL-48861), but nothing that seems to fit our experience of this issue.