26 to 31 character OpenVPN interface names cause gateway names to exceed 31 character limit
If you make an OpenVPN interface name with 26 to 31 characters (in my case, I made them with exactly 26 and 31 characters), _VPNV4 and _VPNV6 are appended to them when making the gateways under System / Routing / Gateways. Trying to add a monitor IP is impossible because the 31 character limit is exceeded and attempting to change the gateway name to honor the limit is not allowed. Working around this requires picking shorter interface names.
I am running 2.4.4_2. I have not tried newer versions to see if this has changed.