Random MACs cause issues with ConnMan as the 'service' identifier is derived from the MAC, e.g. ethernet_b827ebb5e9a8_cable so randomly generated MACs mean a new (unconfigured) NIC on each boot.
That said, I would still expect the second NIC to be detected and brought into an UP state, and for it to receive an IP via DHCP, which will mean you have two NICs in the same subnet etc. - but that's generally not an issue, and you can add a static DHCP reservation in the router to avoid manual configuration on the LE side.
See if this works to assign a static MAC:
nano /storage/.config/udev.rules.d/99-network-custom-mac.rules
Create that ^ file with the content below:
SUBSYSTEM=="net", ACTION=="add", KERNELS=="0001:04:00.0", PROGRAM="/sbin/ip link set %k address ae:7f:66:a5:82:77"
Reboot to see if it works. Check the MAC assigned, then reboot again and check again to see if it changed? .. and if no, has the NIC been detected properly this time?