DDNS on OPENVPN iterface does not update

[Fabio72]

I have a couple of DDNS services registered for OPENVPN interfaces.
When my OPENVPN public ip changes (restart of openvpn client, WAN flapping…) my DDNS registrations remain invalid, while the WAN DDNS get renewed.
I have to force renew them to have valid DDNs again.

Both vpn clients are in a gateway group but I don't think it matters.