I've been having issues with this for a while and reading through the other posts on this issue has not helped unfortunately. When I attempt to use 1Password with the Qantas Frequent Flyer login it only fills in 2 fields and not all 3. I have tried several attempts with the "Save new Login" facility and it does resolve the problem (btw, I think this feature is broken).
I'll attempt to describe the issue here in as much detail as possible. The Qantas FF login page (https://www.qantas.com/fflyer/dyn/program/welcome) has three fields the need filling in, viz. memberId, memberSurname & memberPin (used web inspector to determine these). 1Password has values for all three of these under "Web form details". It also has values for username & password which you can't delete. It seems that username is linked to memberSurname & password to memberPin. When one navigates to the site the memberSurname & memberPin fields get populated but the memberId never does and needs to be entered in manually.
Any help with this is much appreciated.
1Password Version: Version 6.7.BETA-1 (670001)
Extension Version: 4.6.3b3
OS Version: 10.12.3
Sync Type: iCloud