I had a look at this today and in fact, I can’t reproduce reliably. I tested with an old passkey on meta on a separate account (it worked) and tested with an old passkey on a second instance, it also worked. My first test was unreliable, likely because that passkey wasn’t used correctly.
As you note, a new passkey registration also worked.
I wonder if this is the case, hopefully we get some feedback from other Windows users soon.