Go to Tools > Preferences > Logon History and check if the "Store with password" option is enabled. If it is, check if the user@database is listed in the "Fixed users" list. If it is, remove it.
I cannot reproduce any problem, except when you add a user to the "Fixed users" list that is also in the "History" list. If they are in the fixed list, they must be specified with password (user/password@database), otherwise you will be prompted.