• Add controls to ribbon bar and save this pointer (result of Add)
  
• Run app and resize application frame to minimum width
• Now, the single controls should be groupped by parent button
• click this button to popup/see your controls
• Click on some button

If you now using your saved pointer, you see that this points to a freed memory block.

The reason is, that you frees the original controls!

                // we need to copy the popped ribbon group to the existing (in order not to lose user's changes)

                CXTPRibbonGroupPopupToolBar *pPopupToolbar = DYNAMIC_DOWNCAST(CXTPRibbonGroupPopupToolBar, m_pCommandBar);

                if (pPopupToolbar && pPopupToolbar->m_pRibbonGroup)
                {
                    m_pPopupGroup->RemoveAll();
                    m_pPopupGroup->Copy(pPopupToolbar->m_pRibbonGroup);
                }

I think, this problem is not implement correctly. Controls should force to move, not copy/duplicate, in other popups. It should be transparent where they should be displayed. It's the same control (e.g. button)!