It mayn't help, but i was having the same problem, specifically with facebook, and found that one of my house guests, in not wanting to save their password on my laptop, clicked "never" when opera asked if she wanted to save her password.
This seemed to have exempted facebook.com from using password manager at all.
Solution for me was: clicking "Settings" (Alt+P) in the Opera menu tab --> clicking "Privacy & Security" on the left --> under "Passwords" clicking "Manage saved passwords" --> and removing "facebook.com" from the list of "Never saved" passwords.