The CLI statement interfaces openvpn vtun10 openvpn-option '--tun-mtu 1500 --fragment 1300 --mssfix' will render in vtun10.conf to
--tun-mtu 1500 --fragment 1300 --mssfix
On startup OpenVPN complains about:
Apr 26 09:26:47 vyos openvpn-vtun10[27854]: Options error: Unrecognized option or missing or extra parameter(s) in vtun10.conf:76: tun-mtu (2.4.7)
This happened to work in 1.2.4 so it must be something with the rendering - I know that passing openvpn-option is maximum bad practice but it should not become a problem on an upgrade