This is a strange one that I haven't figured out just yet. It doesn't seem to be related to it being a crosspost. Was it displaying video controls (play button) but not playing, or did it just render as an image with no controls?
I have stumbled across similar issues, but I can't consistently reproduce the issue for testing. I'm thinking it has to do with the video player failing to load the video, and then not retrying. For a while I was thinking it may be an issue causing video controls to show up on image posts, but now I'm thinking it's just videos failing to play.
I'll look into this further and see if I can't figure out what is going on.