Appreciate the assistance on it all. This is only a homelab turned private cloud setup, but XCP-NG has been a pleasure to work with, thanks for all you and the team do!

So I'm not 100% but I've resolved the symptom.

I have 3 VLANS involved

DATA - Endpoints
PRIV - Reverse proxy manager exists here
MGMT - Where XOA exists.

Myself and users access XOA via the reverse proxy, which access the webserver in the MGMT vlan.

My hosts had their primary management access on the DATA vlan, a holdover setup from prior, and a secondary management access on the MGMT VLAN and a third for data migrations.

XOA was connected to the MGMT IPs of each host.

Watching packet capture I see packets moving from the originating endpoint in DATA, to the reverse proxy, from it to the XOA backend server. Then suddenly I see XOA in the MGMT VLAN try to access the hosts directly in the DATA VLAN which was blocked.

Problem is, it seemed to send packets meant to originate from the MGMT-VLAN as sourced from the DATA-VLAN.

The resolution was I just removed the secondary management IP and switched the primary management of each host to the MGMT-VLAN alongside XOA.

They're happier now at least.