At now KDE su shows popup window with Password dialog, that contain full text of command to execute. But this text is not selectable via mouse, so there are no ways for copy it to clipboard and check this command manually in console. Please allow selecting command text via mouse for easy copy-pasting.
Should just need setTextInteractionFlags(Qt::TextSelectableByMouse | Qt::TextSelectableByKeyboard); set on the label, as in https://cgit.kde.org/kio.git/commit/?id=025e5e9957ff92c8196f820e9490f337ebfbe317
*** This bug has been marked as a duplicate of bug 237182 ***