There's an issue with 1Password for Mac currently regarding the display of the password strength.
1. Create a Login with a strong password so that the 1Password indicate correctly that the password is strong
2. Delete the password field using show web form details
3. Save the item
4. Create a new password field using show web form details and set the password during this dialog to something simple like 123
5. Save the item
The color for the password strength is red as the password is weak
The color for the password strength is green.