Nevermind... I think I saw it... checking now....
I had made a test edit to /etc/config/openvpn to use a different push for IPV6 and I had forgot to add the "list" keyword: as a result, the uci config was invalid and it was being skipped by Luci.
Openvpn itself simply doesn't see the uci config and I failed to notice the missing route.
@anon50098793 thanks for the help, yet another case of having to explain it to someone else helps finding the problem 