I found that one of my albums was not found during scanning. The folder name ends with a dot (Chef’Special - One for the Mrs.)
After unsuccessfully trying tips such as touching file dates or moving into parent folders, I renamed the folder by removing the trailing dot and the folder and files were picked up properly by Roon upon a forced full rescan.
My files are on a QNAP 871 NAS by SMB. Roon core is on a Mac Mini with El Capitan 10.11.6