I am experimenting with an AU effect of the type kAudioUnitType_MusicEffect that supports any possible bus layout in Logic Pro. And it works as expected when the effect is used in an Audio FX slot. For example, if I instantiate the plugin as 5.1, Logic will correctly set both input and output bus as 5.1 Surround.
However, kAudioUnitType_MusicEffect (aumf) can also be placed inside the instrument slot as “AU midi-controlled Effect”. In that case, it receives audio through the sidechain. Logic uses the first input bus as the sidechain bus in that case. Problem is that, apparently, sidechain in Logic can only be a single mono or stereo bus, not surround. As a result, Logic will set input as Stereo and output as 5.1 Surround in this case, and not both to 5.1 as expected.
Sidechain channel limitation is not a problem for regular use of sidechain in Audio FX slots (e.g. in a compressor), but it is can be a problem when using a surround effect as an “AU midi-controlled Effect” as, in that case, sidechain effectively acts as the main input.
I guess this is an inherent limitation of Logic, nothing specific to JUCE, right?