AU builds with "Legacy" build system but not "New" build system - why?

If I build an AU plugin with Xcode’s “Legacy Build System” it works - but if I select “New Build System” the build fails with “Command Rez failed with a nonzero exit code” error (that stems from IPlugAU.r).

Why is that? Is there a way to fix or update the IPlugAU.r code for the New Build System?

This does not seem to be an issue with VST2, VST3 or AAX - they build OK using “New Build System”.

BTW - I already checked this: and I have the path as shown - but still get the Rez error with New Build System.

I found my problem.

I have been using a low-level “helper” code for denormalizing that worked really well in the past but apparently is too old now. I commented out that code and the project builds as New Build System with no errors. This denormal code was also causing the arm64 build problem on this thread: Successfully compiled for ARM/M1 - I thought!