Unable to import .abbu file into Contacts on Mac OS 15.2

Any suggestions how I can open regain access to my contacts?

A few weeks before I updated my MacBook Pro to Catalina (OS 15.0), I backed up my Contacts by exporting them as an .abbu file. I then opened the file and verified the backup process worked.

In 15.1 and now in 15.2, however, when I go to Contacts > File > Import and select that file (as Apple instructs), no contacts import—just the name of my sole smart group. Same happens when I double click the .abbu file. The situation also occurs when I used a version of the .abbu file backed up three months ago via Time Machine.

(Also, I don't use sync them to iCloud or another device, so there's no device from which I can sync them over.)

The .abbu file is 229 MB too, so it clearly isn't empty.

Any ideas how I can use those files to restore my contacts?

Ctrl-clicking and choosing Show package contents opens the folder at the end of this post (the Images folder is empty and Meta has one .abcdg file—but Sources has three subfolders, each of which has a file structure similar to the the main folder's).

Also, when I search for the name of a person using Spotlight Search, it returns his or her Contact card from which I can copy and paste information. Is it possible Spotlight is pulling that info from the aforementioned .abbu file or a different source altogether? (Every name I search for in Spotlight returns two identical results too.)

I tried a bunch of tricks that worked for other users here, but none worked for me.

I also ran through some troubleshooting options with Apple support and quickly got bounced up to a level 3 advisor. She ended up forwarding the issue to engineering after trying to import the contacts on a test accounts and a new hard drive partition failed. The advisor and I performed some tests and sent the logs to engineering. Its response was to perform more tests.

Thanks!