@theaccountant - if you did not 'copy' all the custom columns then if the 'old' opf files were copied then they would contain junk data, hence they're not copied.
I think you'll find that 'new' junk free opf files are being created in the background - look at
Library Maintenance->Library metadata backup status.
Ah-ha - I see you already found that
Maybe they're never copied, custom column select-ability is a fairly recent new feature, Kovid may have decided that opf recreation would be a safer option in all cases.
BR