To reproduce:
1. Create new Qt Widget project GUI.
2. Autocreate MainWindow widget
3. Now we have 3 files: main.cpp mainwindow.h mainwindow.cpp
4. Setup UTF-8 codec for all editor files in preferences.
5. Open mainwindow.h and type comment in Russian 'ПРИВЕТ из России'
6. Close mainwindow.h
7. Open main.cpp
8. Refactor MainWindow with context menu item (or press CTRL+Shift+R)
9. Change class name MainWindow to MainWindow1 and press Replace button.
10. Open maibwindow.h and see yellow waring: Can't convert mainwindow.h to UTF-8. File is readonly.
11. All russian letters 'Р' in upper case are '? '. Other letters are ok.

P.S. If we do not close mainwindow.h (stage 6) - all ok with russian letters.