Unresolved Cython References in Pycharm Professional

I recently upgraded from Pycharm Community to Professional on my Mac. However, none of the Cython references are being resolved. Even a basic cdef int x is unresolved in a .pyx file. I have worked through a long list of possible unresolved reference fixes (https://stackoverflow.com/questions/11725519/pycharm-shows-unresolved-references-error-for-valid-code), however none works for me.

I would expect Cython support to work on a fresh install of Pro. I suspect there are some residual Community files somewhere that are causing this issue. Any ideas?

The Cython scripts compile and run fine, but the unresolved references are inconvenient. I don't have this issue on my Windows installation, but I have always used Professional on that PC.

Currently using Professional 2021.1, build #PY-211.7142.13 running on MacOS Big Sur 11.2.3

Thanks