QLineEdit in QComboBox for InputMethod with Japanese Keyboard doesn't go well

On only the case of QLineEdit in QComboBox, I couldn't switch between two modes.(for example, from english to Japanese IME Method). Other QLineEdits has no problem.

It does not mean that I cannot change keyboard of one language to  keyboard of another language.I only use Japanese Keyboard. Japanese Keyboard can normally switch between English keyboard & Japanese Input Method.I cannot switch between the two in the case of QLineEdit in QComboBox.

I have tried to another machine , windows 10, but the same problem happened.