Interesting. Just one data point, but my wife and I both have airtags on our keys. When I drive her car (and she's not in it), I get a notification about her keys traveling with me. We're on the same iCloud "family" account. I wonder if there’s something else that needs to happen?

When you go into "Find My", can you see your wife's devices and keys?

I see all her other devices, but not keys. Now that I know how it should work, going to see if re-registering the keys changes things.

I would reach out to Apple about that. It’s not supposed to work that way.

“Okay, so what we’re going to try and do here, is reset and erase, and not restore from iCloud backup, so we can rule out this being something in your profile.”

When you say "iCloud family account", do you mean you both log into the same account using the same credentials, or that you use "family sharing" to, e.g., share iCloud data, purchases, etc.?

Not the same credentials, the other one. I forget what they call it, but basically we pay one bill and share iCloud storage and apps between us.

Yes we have some kind of shared account or family/group, e.g. with shared iCloud storage and so on.