I'm trying to do the same thing to add an inputBus to an AU v3 instrument in order to get a Sidechain dropdown menu in Logic. I'm also having trouble, did anyone every figure out how to do this? I was able to reproduce the same behavior as user "jfjs". As far as I can tell this is the same boilerplate as in the AU effects example posted by user "NB System".

I just noticed this problem too when I upgrade my phone to iOS 15.3. All the usdz file I have on my phone now only display in "Object" mode but not "AR" mode. This is really big regression, please let us know when the issue is addressed. Just a random guess, maybe somehow AR Quicklook doesn't have permission to access the camera anymore after some conventions changed with the OS update?