I posted this issue 4 months ago. But there are so many caveats, like Lemmy.World still hasn't updated Lemmy past software version 0.19.3 so the bug doesn't even happen on it (having been introduced later), whereas StarTrek.website has the latest 0.19.9 and it does auto play - see e.g. this example video post). The GitHub issue looks like it was simply closed, pointing to another one that says that it is fixed, except ofc we can clearly still see the behavior happening. It also happens on Mbin as well, more or less.
Edit: okay so clicking the above example does not trigger the behavior, plus shows me as logged out for some reason. Try this search link, which for some reason retrieves the post twice (database weirdness issue? It took forever the first time to find it btw, so that does seem likely?), but clicking either one auto plays for me with sound, only when logged in with a Startrek.website account, and in Firefox on Android.
I have since switched to PieFed bc I like the features here much better than Lemmy, and e.g. this issue does not happen here (edit: although here is an example of an embedded YouTube and piped video post - note it waits to be clicked to play). If you want to keep your Lemmy account though, you might try switching to an app that you like better. Thunder and Voyager are great ones, and I've heard wonderful things about Sync and Connect.
Lemmy only updates things extremely slowly (example). Thus, very few people seem to prefer using the basic web UI, as opposed to apps.