I just tried and it's working fine for me on beta 11.
Either it's a caching issue, in which case you should clear the Flarum cache (command line or clear cache button on admin dashboard), and also maybe Cloudflare cache or similar if used.
If that's not it, then the 3-dots menu disappearance could be caused by any extension that has an issue. You could try disabling extensions one by one until the menu reappears in order to find the culprit.
There might be a javascript error message in the console of the browser development tools as well.