This is four years old but it may still be relevant regarding Sidechain: Sidechain is not silent as expected (AU, Logic X 10.2.2)
I’d also be curious to know if things haven’t changed since, are you testing the AU with Logic?
Perhaps you can test it with some other DAW (REAPER or JUCE’s own audio plug-in host) to sesee if you get the same behavior in AU format (but not the others)