I tested Stock OS and CalyxOS for my Pixel 8 Pro and emby works flawlessly for tv shows.
Now this is where the weird stuff happens when watching on GrapheneOS. When watching a tv show it will just randomly skip episodes, sometimes even in the middle of watching one. And not only that it will skip multiple episodes and then land on another episode and play.
Example: Playing Big Bang Theory S07E02 till the end then click "Next Episode" Then S07E03 plays fine till midway through and skips episodes automatically.
From what I've seen and tested (This may or may not be right): I see that GrapheneOS implements it's own webview called Vanadium Webview. This could be related to the issue I am having.
Also I have tested Emby (Google Play) + Google Play Services and Emby (APK from website) + Without Google Play Services. If anyone can check into this that has emby that would be good. Best test would be to watch 3 episodes and "skip intro" and "next episode" that way it follows my testing.