When changed the project specific editor settings (see attached screenshot), QtCreator creates UI files with or without UTF-8 flag. In the attached project, one UI file is created with default encoding set to UTF-8, the other is created with default encoding set to SJIS. QtCreator cannot open both of them in one setting.
Encoding of UI file can only be in UTF-8. It does not make sense for QtCreator to create different types of UI files according to encoding setting. It is worse that QtCreator choose to open one but not the other even though both files are actually in the same encoding.