My Naim NDX-2 <> AppleTV handover and grouping is flawless now:
It works starting with either device and then switching to the other, with and without stopping the first device, and with and without waiting 5 seconds. Grouping also works starting with either and grouping while stopped as well as grouping while playing. (I realize now that the one thing I didn’t test was grouping a device that was already playing something different, but I trust you 
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