Content of selection buffer is updated when mouse is moved with pressed left button, but not updated when using key combinations like Shift+End (select up to the end of line) or Shift+Home (select up to the of line)

How to reproduce (using Qt Creator as example, but happens in other Qt and KDE applications)
1. Open any source file in Qt Creator on Linux (or possibly other OS using X11)
2. Make a selection using mouse movement
3. Check a content of selection buffer (press middle button on empty line) - it matches the selection you've made before
4. Select part of another line using Shift+End
5. Repeat step 3 - see that selection buffer was not updated
6. Repeat all steps in GTK application - works ok (I've tested in Geany)