Hi VWB,
Thank you for getting back to me and confirming that it was a live webcast.
Based on what you described, this behavior is expected in a certain scenario. If you share a recorded video, the video would restart from the beginning when a participant logged off or was disconnected and then logged back in. Once the webinar transitions back to the live broadcast, the video would end, which may appear abrupt or cut off.
If you were presenting and screen sharing, it would not start over from the beginning if someone logged off and logged on.
I understand how this experience may have been confusing and apologize for the inconvenience.