OpenVPN site-to-site server certificate verification failing when using an external PKI

Tactis

I've run into an issue trying to configure peer-to-peer (SSL/TLS) when using pfSense as a subordinate CA to an ADCS (Active Directory) CA.
Are there any caveats or reasons why using an external root CA doesn't work in this scenario?

ADCS CA is created
ADCS creates a subordinate CA, the CA cert & key imported into pfSense at site A.
Site A created a server cert signed by the above CA and uses it on an OpenVPN peer server.

When trying to connect, the client pfsense at site B always fails to connect with TLS Authenticatino Error. From the logs:

VERIFY ERROR: depth=1, error=self signed certificate in certificate chain: C=…
OpenSSL: error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed

The issue seems to be the same with any of the processes below:

Site B imports the ADCS CA and pfsense subordinate CA from site A seperately
Site B imports the ADCS CA and pfsense subordinate CA as a chain
A user cert(signed by the CA) is generated in site A and exported and imported into site B.
A CSR for user cert is generated on site B and signed by site A
The tunnel uses the ADCS CA as the peer certificate authority
The tunnel uses the pfSense subordinate CA as the peer cert authority.

I instead exported the internal self-signed CA from site A, create a user cert and imported the CA and Cert into site B and it works. After some searching it seems that others have had the same issue.

This is the guide I've been using, I have a lot of experience already configuring shared key site tunnels and OpenVPN remote access servers, just Peer & PKI is new to me.
https://docs.netgate.com/pfsense/en/latest/vpn/openvpn/configuring-a-site-to-site-pki-ssl-openvpn-instance.html