PPTP VPN connections suffer from the same problem.
I have an up-to-date Debian testing system with KDE and XFCE desktops
installed. PPTP VPN connections created using the network-manager-gnome
systray icon in XFCE behave in exactly the same way as PPTP connections
created using the plasma-nm systray icon in KDE: the new connection can
be seen from both desktops (regardless of which was used to create it)
but only works in XFCE.
When I connect using the network-manager-gnome applet in XFCE, a VPN
password dialog appears and everything works fine; when I try to connect
using the plasma-nm applet in KDE, no password dialog appears, the
plasma-nm systray applet tells me that it "needs authorisation", and the
connection fails. Sometimes, if I look very carefully, I think I can see
a password dialog appear very briefly, but it disappears so fast I
cannot be sure.
It looks as if plasma-nm has forgotten how to ask for VPN passwords.
Matthew