Thanks for your patience with this issue, I’ve now pushed some changes which should partially resolve the issue:
It looks like adding a single output bus to a MIDI FX AudioUnit allows the samplerate to be detected correctly in Logic. Making the same change in the AUv3 wrapper allows MIDI FX AUv3 plugins to correctly determine the samplerate in AUM. Adding an output bus to an AudioProcessor allows the samplerate to be detected without any further changes to JUCE code. However, this setup isn’t particularly ergonomic (we can’t easily warn users that they need to add an output bus, and conceptually the bus shouldn’t be required in the first place). In an attempt to make MIDI plugins “just work”, I’ve adjusted the AU and AUv3 wrappers so that MIDI FX will always have at least one output bus, even if no buses are requested by the AudioProcessor.
The changes on the develop branch should resolve sample rate detection issues in AU plugins, and with AUv3 plugins on iOS.
Unfortunately, it seems like Logic always passes a samplerate of 44.1KHz for MIDI FX AUv3 plugins regardless of the project sample rate, so this combination remains broken. I don’t think this can be worked-around in JUCE.
As always, please let us know if you encounter any issues with this change.