Assigning the shortcut shift+ctrl+U to Format > Double Underline replaces the selection with an underlined U. Everything works well with shift+ctrl+W.
Issue happens for the planned change in https://gerrit.libreoffice.org/#/c/45150/ for bug 113939 on 6.0 as well as LibO Fresh with manual configuration.
Double Underline enables Underline in menus and toolbars, so the functions seem to depend on each other.
But i tested the shortcut in ubuntu 17.10 (gnome 3) and the shortcut wasnt pre-assigned to anything.