Attendee's missing from on-demand webinar attendance reports

We use the attendee reports to validate attendance at our on-demand webinars and are finding that from time to time individuals are missing from the attendance report. We can confirm an individuals registration, but they never show up in the attendance report. If we look in other reports, such as engagement we can see that these people have completed the exit survey and in a timescale that correlates with them completing the on-demand webinar.

Is anyone else having the same issue - registered individuals missing from the attendance report for on-demand webinars? Any ideas why this might be happening? 

We have no issue with our live webinar attendance reports. We have checked back in time and the missing individuals never appear in the attendance report so we know that it is not a lag/timing issue between the on-demand webinar being conmpleted and the report updating. The individuals involved are registering and watching other on-demand webinars at different times/days and showing up in the attendance reports just fine. This appears to happen at least once a month now to different people attending different on-demand webinars on different days with no obvious cause.