All logins from the app correctly appear in the browser extension, but none are usable

Hello!

Yes, manually adding a specific domain to a given Login entry does allow it to appear. The problem is that the number of combinations of domains and already-existing Login entries makes manually pasting all those in rather unfeasible.

Can you describe to me the steps taken to initially save these logins?

They were captured in the 1Password native MS-Windows app directly, before the browser extension was ever installed. (How that was done I'm not sure about, since it was before my time. Possibly one of the then-sysadmins did something clever with imports?)

Personally, I would be totally fine with a Login that didn't contain a matching website not appearing on the dropdown in the username/password text field by default; that makes perfect sense and would be correct for nearly every situation. But not even allowing the Autofill button to appear in the toolbar window's list of every Login on the account seems almost gratuitously restrictive, and impedes the "edge case" use of the software.

I can at least still drop the toolbar window down and manually "Copy" the username and password... although since the browser extension doesn't then offer to update the saved Login entry with the current website domain, this method does not help solve the problem going forward.

I'll probably play around with exporting the account to a .1pux format, unzipping that, and scripting a pass across the data file to insert the appropriate domains to the matching Login entries. (If I re-import the result, will the MS-Windows app complain about duplicates of existing entries, or will it overwrite the existing ones with the updated entries being imported? I guess deleting the existing ones would avoid that... now I'm just thinking aloud.)

If that latter approach works, I'll be very grateful that you allowed import/export of the data!