based on this post in our forum , openvpn seems to deprecate this option --cipher move to --data-ciphers
https://forum.vyos.io/t/some-openvpn-server-cipher-options-are-ignored/11878
in our config-file should add this new option :
https://github.com/OpenVPN/openvpn/blob/master/doc/man-sections/cipher-negotiation.rst
data-ciphers AES-256-CBC
it was added 2.5 based on documentation