I have been having the same issue. I couldn’t even cast my synced content on my phone to my Android tablet. I did get it to work now but it required an extra step. If I enable “Advertise as Server” on my phone and then I press the cast button and connect to my tablet then I can go to my tablet and browse the synced content synched to my phone. As soon as I stop casting on my phone to my tablet my phone shows as offline to my tablet in the “Locations” dialogue. It’s like the Advertise as Server doesn’t really work until I specifically tell my phone to cast to my tablet. For some reason this is a bit intermittent, as I mentioned that I could not even cast to me tablet earlier. It is working at the moment.
Note, when I do play content synched to my phone on my tablet, Plexpy does not show it as playing, not does Trakt, since technically the server is my phone. Oddly enough, if I browse media synced to my phone and play it on my phone, Plexpy and Trakt register the play, even though technically the server is still my phone.
Plex is definitely doing some screwy things with synced content lately.