Like described by someone here:
https://bbs.archlinux.org/viewtopic.php?pid=1494420

When connecting to a VPN connection using KDE's networkmanager applet,
NetworkManager creates a new connection, named tun0 (or tun1, tun2, etc, if VPN
connections are already established). This temporary(?) tun* connection isn't
removed upon VPN disconnection, moreover, NetworkManager creates more
"temporary" connections with the very same name, and after several times of
using VPN connections, I will have tons of connactions named tun* and it's hard
to find my deliberately created connections amongst them.

However, I created a workaround for this problem by writing a script called
/etc/NetworkManager/dispatcher.d/tun with the following contents:

#!/bin/bash

[[ ${1::3} == tun ]] && [[ $2 == down ]] && /usr/bin/nmcli connection delete $1

exit 0