The newer versions of OpenVPN require any push argument to be surrounded in quotes if it's more than one parameter that's part of the push.
As a result adding custom push options via set interfaces openvpn vtunX openvpn-option is completely broken since quotes are not allowed inside.
I would suggest one of two things, fixing the quote issue - or instead offering a set interfaces openvpn vtunX push-options <option>that will put the value inside of quotes when written out to the configuration.
so set interfaces openvpn vtunX push-option "keepalive 1 10" would result in a line in the openvpn config being push "keepalive 1 10"