@Sammie
Firstly, MT does not support TLS Auth.
tls-auth ta.key 1
Secondly, I had the same problem with OVPN and with Version 7.3 It has been fixed, you can check this post, both sides of my tunnel are MT.
OVPN Server
/certificate settings
set crl-download=yes crl-store=system crl-use=yes
/system clock
set time-zone-name=" "
/system ntp client
set enabled=yes
/system ntp client servers
add address=time.cloudflare.com
/ip address
add address=10.10.13.1/24 interface=bridge network=10.10.13.0
/ip pool
add name=OVPN ranges=10.10.13.5-10.10.13.240
/ppp profile
add change-tcp-mss=yes dns-server=10.10.13.1 local-address=10.10.13.1 name=opvn remote-address=OVPN use-compression=no use-ipv6=no use-mpls=no use-upnp=no
/interface ovpn-server server
set auth=sha1,sha256,sha512 certificate=OVPN-SERVER cipher=aes256 default-profile=opvn enabled=yes keepalive-timeout=30 max-mtu=1450 port=443 protocol=udp require-client-certificate=yes tls-version=only-1.2
Certificates
/certificate
add name=OVPN-CA-template common-name=vpn.domain.com key-usage=key-cert-sign,crl-sign
add name=OVPN-SERVER-template common-name=ovpn.domain.com subject-alt-name=DNS:ovpn.domain.com,IP:aa.bb.cc.dd
add name=OVPN-CLIENT-template common-name=ovpn.domain.com subject-alt-name=email:hostmaster@domain.com
sign OVPN-CA-template ca-crl-host=aa.bb.cc.dd name=OVPN-CA
sign OVPN-SERVER-template ca=OVPN-CA name=OVPN-SERVER
sign OVPN-CLIENT-template ca=OVPN-CA name=OVPN-CLIENT