@fabian or @jules Could you have a look at this issue, please?
I just updated to JUCE 4.2.3, and I still get (1-in, 6-out) as one of the 4 (not 3) possible plugin configurations.
The problem still also exists with the JUCE audio plugin demo, if you add {1,2} as explained above.
This problem had been fixed on Febr. 4th, but is back now.
By the way: are there other hosts where it is easy to see which configurations are supported by the plugin?