videos no longer display properly

Just recently I have noticed that videos embedded via the Gutenberg video block are not displaying properly in published posts. They display OK in edit view, and they display properly for about 2-3 seconds in the published post the first time they are played. However they freeze and can’t be activated after that, and the video is replaced by a much smaller default graphic of some kind.

I disabled all my plugins – didn’t change the behavior
I created a test post (the one linked above) and inserted two videos. The first one is the one I was inserting when I first noticed the problem, and the second one is a video from a previous post that continues to display properly in an older post (https://www.fpaynter.com/2020/11/wall-tracking-trials-using-office-sandbox-part-i/)

I have searched the forums, but couldn’t find anything particularly relevant, except from years ago.

If it is relevant, I am using a child of the Twenty-Twelve theme, and I believe I have kept the theme properly updated.

Any ideas?

The page I need help with: [log in to see the link]