This bug can be detected by  creating simple qt quick project, with only main window and text area inside it. After inserting into the text area  any kind of emojie ( it doesn't depend from which source), then after moving cursor somewhere else (or unfocusing the input field) the emojie will be recognized as a preedit text and it will be underscored for a moment (on some devices the emojies are fully redrawn, what makes this bug more detectable). This is definitely a bug, because emojies are not supposed to be composed at all by the the input method. One line Text input component doesn't contain this bug.