While testing the pypi implementation, I found that searching on custom fields only found results (of either case) if I searched using lower case.
For example, "apple" will find "Apple", "apple" and "APPLE" but "Apple" will not find "Apple".
This was decided out of scope for PyPI and a general search issue. I do not recall I've seen in any other format however. In fact, NEXUS-8879 speaks to the reverse.
This is especially confusing because as far as I can tell all the PyPI classifiers start with capital letters. This renders this search custom field useless unless you know this workaround or are lazy about case.