Thank you for the quick response and I apologise for the imprecise description of my problem.
The COSName CIDSet is not contained in the font descriptor. There is data in the cmap which, based on its ID, suggests that it could be CIDSet-relevant data. All my attempts to access this data directly have failed. Is it even intended to access the CID mappings? https://imgur.com/a/kni3c4y
