I can't import contacts into my Addressbook on Catalina

I have a .abbu package from my previous iMac, and I'm trying to import it to Contacts on this Catalina iMac. I double click the .abbu file and Contacts asks me if I'm sure I want to replace my Contacts data with the Contacts from the .abbu file. I click Replace All and Contacts promptly stays completely empty.

I can look inside the .abbu package and find the individual contacts files in there (in Contactsxxxxx.abbu/MetaData/*.abcdp) and quicklook those to see my contact's information, and the quicklook window has a button to Add to Contacts, but when I do that nothing happens as well - Contacts stays empty. I've had this problem for a while on this iMac (going back to when it was new last year) and I've made do by quick looking on the individual files, but with a few days of downtime I'd like to solve this one.

I've also deleted the ~/Library/Application Support/AddressBook folder completely, and it's just recreated empty when AddressBook starts up again as expected. I've completely replaced the contents of the same folder with the contents of the .abbu package, but they're immediately overwritten with an empty version when I next start up AddressBook.

I looked in Console.app and saw a note about sandbox permissions, which worried me - so I put the .abbu file on a different drive without permissions and I watched the contacts actually import, then promptly delete themselves to return to an empty contacts list. Infuriating.

Any hints on what's stopping AddressBook from working? I can make new contacts fine, but the instant I try to import my older contacts it of course deletes anything I create there - just never brings in the actual old contacts list