It appears that on Mac, Directory.EnumerateFiles, when called with a search pattern, behaves in a case-sensitive manner. Deep in the bowels of the Mono sources on github, it seems this may be controlled via a #define of IS_PORTABILITY_CASE and others.
As a workaround, list everything w/o a search pattern and filter yourself, if you care to have a case insensitive search occur.
Seems this has also been mentioned here:
It looks like IS_PORTABILITY_CASE is not initialised correctly. We'll probably have to read some HFS+ as it can be both case sensitive and insensitive.
The easiest workaround for you is to set MONO_IOMAP=case