Thank you for the followup. To answer your queries:
Extensions: yes, I see Capital One Shopping, and Zoom Scheduler
Privacy Guard was turned on (this may be the culprit...)
In Security and Privacy Center: Web Shield, Password Manager,Extension Guard are enabled.
ps. I just turned Privacy Guard off, and now the same video I had cited plays all the way through! So, I will leave it off. I didn't think Privacy Guard was the culprit initailly, as it lets the video plays for 5 or 6 seconds before it locked. However, now that is is turned off, it plays all the way, so I will leave it off..
Thanks for your suggestion!