I have the same annoying problem for more than a year. I stopped using Opera just for this matter, cause I need to sign in to many sites protected by password.
It's an Opera issue cause the Password auto fill works just fine in Chrome and Firefox.
I hope this guys work out this truly embarrassing issue, cause it's something basic. It makes you lose a lot of time
It won't work in every site, including email sites such us Outlook and chrome