Previous rescans of the media folder didn’t generate any significant errors or warning in the log. So, as per your suggestion, I renamed the folder back to Genesis, deleted cache.db and library.db and rebooted the server. This time it found the Genesis albums! Yay!
I don’t really understand why previous rescans didn’t work, unless I changed the name of the Genesis folder, nor why starting from scratch worked. I’d assumed that doing a full rescan would find everything, but clearly not.