This is pretty bad what’s the problem with these videos. 
Every video has the same thumbnail I mentioned on the OP. But how these videos missing? It maybe something with secure uploads… Never mind.
Missing videos on these topics too, that I noticed until now:
- Smooth Scroll Top on Homepage
- Custom embedded replies
- Full Row Bulk Select
- Desktop to Mobile View Banner
Yeah, I think this topic is related: Missing images at Meta.discourse.org