I have trouble getting videos to work in my community devlog posts:
https://itch.io/t/3487239/savanna-sam-flying-giraffe

I first tried to upload videos directly to itch.io, since it says in the UI for the image upload that *.mp4 video files are supported:

However, in the actual file selection screen, mp4 files can't be selected:

I then tried to host the files myself and to embed them in my posts. They show up in my edit preview, as well:

But in the actual post, there's only an empty space showing up.

Am I missing something? Is this a known issue?

If it's a video hosted on YouTube or Vimeo, the correct way is to paste the link on a line by itself (nothing before or after, not even a space), then press enter. The last step is important. Your video should be auto-embedded at that point. If you tried that and it didn't work, then I don't know.