If it's a total mystery, as in you have checked everything and it doesn't make any sense...
Then I suggest you replace the bus and or the compressors on the bus and reroute to the new stuff, as I suspect that one possibility is that it may simply be a routing issue that is "stuck" behind the scenes.
Kind of like you think you pulled a cable out, but SONAR doesn't think you did.
Good luck.
best regards,
mike