This issue happens when I use LibreOffice on Openbox, not on Windows, not on KDE (have not tested it on other environments).
Say I open an odt with Writer. If I select some text and then press ctrl+c, selected text is copied to the clipboard (as it’s supposed to be), but document gets modified (if I close it, Writer asks me to save before exit). No actual modifications are made, though. I have not changed the original behaviour of ctrl+c in my ~/.config/openbox/rc.xml (actually, text is successfully copied).