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).

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.