I'm also noticing this issue. Applying the background color can get the first frame of the video to show, but I can't influence the playing otherwise.

I am noticing the same behavior on Mac OS (desktop) Safari 15. I can get the first frame of the video to show up if a background color is set on the element. The video will not autoplay (despite having the correct attributes), but I did notice an unusual behavior: I can "scrub" the video by resizing the viewport (either resizing the window or resizing the web inspector drawer).