This is expected behaviour: you can use other third-party AUs and many behave the same way. It’s technically possible to detect “None” being selected as a sidechain by checking if Logic passes identical pointers for both buffers. Unfortunately, this is identical to the behaviour of Logic, when the user selects the same track for both sidechain and the main bus I tried various third-party plug-ins and none of them was able to either detect that None is selected - or function properly if the user selects the same track for both sidechain and main input (they simply ignore the sidechain data). This lead me to conclude that this is a limitation of Logic. But I’m happy to be proven wrong.
I needed to make a decision on which behaviour is more important: let the plug-in detect no sidechain being selected - or having plug-ins function properly if the sidechain is the same track as the main bus. I opted for the latter as I believe this is more future proof if Logic ever decides to fix this problem.
In JUCE 4.1, I used to pass the same pointers to the processBlock function. But I re-worked this to copy the data instead as this caused problems with some developer’s plugin if the pointers were identical - especially as the main bus buffers are also used for the main out.
Well the main bus and the sidechain is opened in stereo (so total is 4 channels) if the main bus is in stereo (regardless if the sidechain is mono) Logic seems to open the sidechain as well.