Ah, you nailed the clue.
It was inconsistent because of how I was viewing the video.
If I close with the X on video, everything is fine.
But if I shrink the video with a zoom action (0:22),
it continues to play inside the browser window.
Then if I close it, the audio continues to play.
I’m guessing the short term solution is for us to just disable the pinch and zoom.