Lost Google Authenticator codes after iOS update and backup

Hello,

I have Google Authenticator replicated acc\ross two iphones and an ipad. I noticed that each device had duplicate entries of the same codes (i.e. each device had a couple lists of the same codes). So I went through each device and deleted the duplicate set. I then confirmed on each device that it had the same list of authenticator codes as the others.

After all lists were de-duped. I updated one of the phones and the ipad to iOS/IpadOS 15.0.2. I also backed up the phone at that time.

Now, I went to use the authenticator app and only a subset of the codes remain on each device- and the ones that remain are not even the same across all devices.

My questions:

1. Is there any way to retrieve the authenticator codes from a iphone/ipad backup?
2. What might have caused this so that I can prevent this in the future? This seems like a very sensitive or buggy feature - or the interoperability of the codes across devices, combined with icloud backup, is buggy.

Any help would be appreciated. I've already read the Apple support pages and I don't think I can restore. I'm puzzled as to how this could have happened.

Thank you.