[U-Boot] ITEAD Board (A20) fails to communicate on boot unless UART is connected

I'm trying to resolve a strange networking issue with the ITEAD Core EVB + AW2041 module.

If I connect all 3 UART wires (GND, RCV & TX) between the board and a USB UART device: I get an IP 100% of the time. If I connect only TX & RCV: I get an IP 100%. If I connect TX & GND, I get an IP 100%.

But, if I only connect RCV and GND... I do not get an IP.

Using WireShark on the DHCP server, I see no traffic is generated by the device.

If I connect the UART after booting, it will not work, even if I send DHCPCD commands, or reboot.

The only thing that seems to work is if I apply power while the UART is connected.