Xcode "IntelliSense" picks up on only 90% of our C++ types/calls

When we build our C++ code in Visual Studio, IntelliSense finds all of the types and functions. When we build in Xcode, it finds about 90%.

There seems to be no consistent pattern to why Xcode skips some things, and then that daisychains into the next header that includes that prior header.

We have a class with If/Else function calls, but Add calls are skipped. Even one header with the struct defined in the same header isn't highlighted as a type within that header.

Sources are built with Gnu makefiles, but ultimately the .o and .d files are all complied and linked together by clang using Xcode 13.3 and we use the new build system. What could we be doing wrong here? This isn't a recent problem, and has happened with all Xcode builds prior.