In the 1Password entry go to the "website" item. To right right there's an "autofill behavior" button. Change it to "Only fill on this exact host" and it will no longer show up unless the full host matches exactly

Is this a per-item behaviour or can this be set as a global default?

I'm guessing this is 1Password 8 only, as I can't see this option in 1Password 7.

I've looked in the settings on 1p8, and didn't find a setting for a global default.

Not entirely true. It can't seem to distinguish between ports..

because ports don't indicate a different host.

Omg thank you, I had no idea they added this feature!