NetworkManager keeps all OpenVPN processes running, even if no vpn is actually
active. Once you activate one, you can't get rid of it in the process list.
Closing the vpn doesn't help. Worse: the processes are kept running even after
all OpenVPN profiles have been deleted:

# ps axu|grep openvpn
 nm-open+  6865  0.0  0.0  47384  7564 ?        S    18:01   0:02
/usr/sbin/openvpn --remote <IP> 443 udp --nobind (...)
nm-open+ 19826  0.0  0.0  47384  7776 ?        S    18:13   0:00
/usr/sbin/openvpn --remote <IP> 443 udp --nobind (...)

At one time I had 4 profiles with 4 active processes.
This causes unstable OpenVPN connections, with lots of TLS errors and
invalidated connections (i.e.: no traffic is possible after a number of
seconds).

# journalctl -f
Mon Mar  4 18:23:56 2019 80.155.9.251:51053 TLS Error: TLS key negotiation
failed to occur within 60 seconds (check your network connectivity)
Mon Mar  4 18:23:56 2019 80.155.9.251:51053 TLS Error: TLS handshake failed
Mon Mar  4 18:23:56 2019 80.155.9.251:51053 SIGUSR1[soft,tls-error] received,
client-instance restarting

After deleting all OpenVPN profiles and killing all OpenVPN processes,  I
re-imported one OpenVPN profile and everything worked flawlessly after that.