Goto the URL opera://settings/passwords and delete one of the entries. Then, goto that site, log out of it if you're not and log back in to have Opera ask you to save the login info. Tell Opera to save it and see if that one autofills. If not, maybe it doesn't have anything to do with the passwords being imported.

Did you import via csv file from Chrome via opera://settings/passwords or did you use the wizard at opera://settings/importData?