I’m having a similar problem - in my case, Watch Together invites only seem to work one way. All of the media is on my server, to which my friend has shared library access. If I send the invite, they never see it (using both a Windows and Android players) on their end. If they send me an invite, for the exact same media on my server, I see the invite appear (on all of my Mac/Apple devices) and it works fine. This used to work in both directions, not sure how or when it broke.