Most don’t load for me. Just get a black screen. The ones I’ve checked were mp4s from imgur. Not sure if that accounts for all of them though.
Edit: I think it started after the most recent update. Edit 2: mp4s from catbox.moe also doesn’t work. It shows a spinner at the bottom but never loads.
I’m getting this issue a lot too, sometimes if you come back out to your feed and back into the post multiple times it will eventually load, other times not.