Multiple Username/Passwords for Single URL/Login

Update / Resolution:
Following contact with 1Password more directly and their usual great support, a solution has been found.

The issue appears to relate to the fact the username field on the input form presents as a dropdown list and not the usual HTML text field.

Additionally, the current 1Password behaviour with this relates to the naming of the field labels in the 1Password login item - more on this below.

When the password is initially stored in 1Password only the password is saved along with it's form field. The dropdown selection does not get stored. The dropdown has a default user though, so this helps.

I have multiple username/password combinations stored in a single login item in 1Password because they all logically relate to the one specific device that is the subject of the login item.

Changing the username/password pair field names (labels) in the 1Password login entry so that they are all unique fixed the problem and the main/default username was left alone in the dropdown, so everything then worked fine.

e.g.

Example original login item:
username: uname1
password: pw1

Section Block A
username: uname2
password: pw2

Section Block B
username: uname3
password: pw3

etc..

Fixed login item:
username: uname1
password: pw1

Section Block A
user two username: uname2
user two password: pw2

Section Block B
user three username: uname3
user three password3: pw3

etc..