You can not have inputs assigned to the main LR mix and assigned to a sub group
that is also assigned to the main LR mix at the same time.
In that case you need to un-assign the input channels from the main LR mix.
It’s the very slight latency difference in the routing paths that when summed together
cause the phase issue.
Other digital mixers do the same thing, some have buss latency routing compensation.
Now if the sub group output is not assigned to the main LR mix but just feeding another
zone, recording, ect. then you do not need to un-assign the inputs from the main mix.
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