Orbi VPN - Random MAC addresses for same device

My experience is different than yours.  I think there could be several issues.

1. MAC addresses do not pass through routers.  When a packet gets forwarded by a router, it picks up the MAC address of the router.  It is the Internet Protocol (IP) address that allows routers to decide where to forward packets.  When I connect to my VPN, the device shows up with a MAC address "00:00:00:00:00:00".  As an example, I just connected my phone to the Orbi OpenVPN and also opened a Hot Spot on the phone.  Then, connected a laptop to the phone Hot Spot and opened OpenVPN on the laptop.  They both show up on the Orbi Attached Devices with empty MAC addresses and a connection type of "vpn".
   Without a MAC address to work with, there is no possibility for an IP address to be assigned.
   So, the first question is, "How does your OpenVPN report MAC addresses when mine does not?"
   
2. The second issue is that VPN assigns IP addresses in a different IP subnet than the primary DHCP subnet.  My Orbi uses 192.168.1.x with subnet mask 255.255.255.0, and I have assigned IP's within that space to many devices and left the remainder for DHCP to use dynamically.  These two VPN devices were assigned in the 192.168.2.x subnet and Orbi has an internal mechanism to allow them to connect to the internet and to local devices.  I think if it were possible to assign the VPN devices a 192.168.1.x IP address, then Orbi would not be able to forward packets to them. And, if the subnet mask was changed to 255.2555.254 that would make it even worse.

In short, I think it is impossible to assign IP addresses to devices which connect via VPN and have VPN function.  What I cannot explain is how your Orbi reports MAC addresses for devices connected via VPN.