sorry for bringing up this old thread. But I have more or less the same issue here. Using 2.2.8, but it happened with earlier software releases as well.
Starting up the 6500 radio and Maestro (via Smartlink) local sidetone is missing. Entering the TX menu, the Sidetone button is enabled. Just by toggling this button to off and on again the sidetone is back. Or by changing the sidetone level.
I can reproduce this behavior by just dropping the connection with SmartSDR on PC or iPad and reconnecting with Maesto. And the sidetone is gone again...
I tried to use a different profile, but no change. Key is directly connected to the Maestro.