Ah OK, now I understand. Then the issue is at the group key 

* [Channel1] sync_master == 1 // Channel 1 is the current sync Master 
* [Master] sync_master == 1 // ?? Who is the sync Master now? 

[Master] is the group for the Master output, not for a Input like Channel. 

Actually you have introduced a new Input like device right? So lets introduce a new group key for it like 
[SyncClock] sync_master == 1 or similar. 
And what about changing "sync_master" to "sync_master_mode" or "sync_is_master"