Since ‘stand alone app’ and ‘AUv3’ are separate instances there might be a need to remap the CCs in the AUv3 Plug-In if it has any CC mapping capabilities?
(AUM again handles this easily with integrated CC → AUParameter mapping capabilities).
Also In Cubasis in order forward all CCs to a plug-in the track apparently needs to be armed for recording otherwise it will not fully process the midi input?! (This feels a bit weird to be honest).
If the AUv3 Plug-In is hosted on an Audio Track in Cubasis there is currently no way to directly forward Midi Input to the AUv3 Plug-Ins which is easy to do in other hosts like AUM.
In Cubasis plug-in automation recording can also be done from the Plug-In UI (Enable W and tweak the plug-in during playback) which is also the only way to automate plug-ins hosted on Audio Tracks.
Does the AUv3 respond to Midi CC’s when used with other AU hosts?
I do not own the AUv3 Plug-In in question so it might be worth to get in touch with the plug-in developer.
Hopefully this will be sorted out but there’s too many ‘loose ends’ to tie up in troubleshooting this.
Cheers!