1Password is filling in the wrong Secret Key

Thanks, Joy. I resolved this issue with 1P support earlier today. The explanation was: the mystery key was hiding in the "auto-saved web details" for the login item, which aren't searchable or even visible until you expose them in the advanced settings.

I knew it had to be there SOMEWHERE, obviously it did actually exist! It just had to be hard to find in some way... and that's where it was hiding. Ta da.

I had long been aware of the auto-saved web details, and I truly believed I had already checked there. But I suspect I made the mistake of assuming that searching for a unique string would find anything saved in those details, and then just failed to actually visually inspect them.

Also, I reasonably assume that data saved there would never pre-empt the same data in the main login item. I can see how it could happen, presumably something about field labelling, the auto-saved item is probably a more exact match for the autofill. But it does still seem a bit janky that a hidden-by-default bit of data could have that kind of power.

Anyhoo, mystery solved!