I don’t know what happened on December 18, 2020, but Vimeo links posted on any Discourse site play without audio when using the iOS DiscourseHub app (at least on iPad, I don’t have an iPhone to test). Also, clicking on the progress bar makes the video permanently freeze.
Videos created before Dec 18 are unaffected by this (I post Vimeo links every day, so I was able to narrow it down to this date).
Here’s an example link without any embed restrictions
https://vimeo.com/508864124/fcbbcc92fa. On desktop it plays fine, but if you use the DiscourseHub app on an iPad, there’s no sound and it freezes if you jump forwards/backwards.
Also, some of my users have reported this behavior when using Chrome and Safari on iPad, but using those browsers works well on my iPad.