Steps to reproduce the issue:
1. Run the attached reproducer.
2. Click the QLineEdit on the main window, and try to enable Japanese IME (e.g. [Ctrl + Caps lock]). Japanse IME will be enabled there. (Screenshot01.png)
3. Click "Window" button. A dialog pops up, and try to enable Japanese IME in the QLineEdit there. Japanse IME will be enabled there. (Screenshot02.png)
4. Go back to the main window, click "Menu" button. A menu window pops up, and try to enable Japanese IME in the QLineEdit there. Japanese IME will NOT be enabled there. (Screenshot03.png)

The QLineEdit, where Japanese IME cannot be enabled, is located in QMenu/QWidget/QHBoxLayout.

With English Keyboards, enabling/disabling Japanse IME can be operated with shortcuts such as [Alt + Backquote (`)], [Ctrl + Caps lock] :
https://support.microsoft.com/en-us/windows/microsoft-japanese-ime-da40471d-6b91-4042-ae8b-713a96476916#ID0EBBL=English_keyboard_(101/102_key)