[ownedbycats]

Google Drive is known to strip the id numbers off the book folders because it's used internally for revisioning. Open the library folder and see if that's the case here.

The best solution is to restore to a backup where the ids. But if that's not possible, my personal suggestion would be to make an entirely new library use 'add from folders and sub-folders' (click 'yes' for multi-file books) to re-add the entire old library. If the OPFs are there, I believe it should copy all the metadata in.

Someone else might have a better suggestion.

EDIT: Unfortunately, I'm not sure if any custom column information will get copied in from the OPF. But if you have any, re-create them with the same lookup names and try.