From Esteban Nicolas Ruiz in the user mailing list:
I have created PDF/A-2b document with pdfbox and I have found that the font subsetting provided by pdfbox doesn't match the following requirement from the specification:
Specification: ISO 19005-2:2011, Clause: 22.214.171.124, Test number: 4
If the FontDescriptor dictionary of an embedded CID font contains a CIDSet stream, then it shall identify all CIDs which are present in the font program, regardless of whether a CID in the font is referenced or used by the PDF or not.
That is: some CIDs which are in the CIDToGidMap don't appear in the CIDSet.
Maybe it is a bug or you can add an option to generate the CIDSet to be consistent with the CIDToGidMap.
Additional details (including a sample pdf file) here: