In juce based hosts, juce based AU plugins, the first call to prepareToPlay, is always with input and output channels == 1 , then after setStateInformation(), the i/o channels will be corrected to the right number.
this happens only when the hosts loads a previously saved project
Yes, this is common behaviour from plugin hosts. You should design your plugin with this in mind :)
I would say its very specific to Tracktion ;-) (and other Juce hosts)
Live and Logic do it better.
I think also it is not intended the way the wrapper/plugin-format was programmed (i think its a bug)
in addition:
some plugins (spatial processors) work different in mono/stereo/surround (and have different settings for every setup)
But the stereo-version/surround has also option, so it can work in mono, so this needs to be a reliable information
So when the plugin is opened (again) as mono (but in the saved project file it was stereo) , internal changes will be done (channel setup), so that the plugin can work in mono.
When its turned to stereo again, redundant changes in the channel-config will be lost.
> You should design your plugin with this in mind :)
Yes currently 90% of my developing time is care about all the different host-behavior, 10% developing the plugin itself (no joke, and i don't do weird things), i hope that juce can help to change the ratio, to have more time for developing the plugin-algorithm