Found what the issue was: by default, the player's playback coordinator uses the asset URL as an identifier to check the same thing is playing on all devices. If you adjust this URL at all, such as to include data to authenticate viewers, analytics, etc., then the coordinator will think different things are playing and playback will not sync. If you have unique URLs you need to override the player item identifier via this delegate method.
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After some digging around, it seems that you need to observer the session properties individually, in addition to observing the session, as per this documentation page. The sessions() sequence only broadcasts new sessions it seems, although it that doesn't seem to be documented anywhere. Wish the documentation made it clearer that you need to observe this properties directly.
Same here
UPDATE:
This thread has a response from Apple: https://developer.apple.com/forums/thread/688320
TLDR; known issue, fixed in beta 7