I'm using the beta of the meetingAttendanceReport for OnlineMeeetings and I've noticed that the data changes and sometimes get lost.
One use case:
I was hosting a meeting yesterday with 4-5 participants, the meeting was closed at 17.30, no report was available 12 hours after the meeting was closed. I figured I'll make sure to close the meeting so I logged in to the link again and clicked "End meeting" in the Teams-app.
After this my 40 seconds spent in the meeting today was available in the API but NOT the participants from yesterday. When downloading the CSV-report form the teams-app there is a complete list of participants.
Please let me know if you need further details.