In D24866#558796, @jgrulich wrote:

I did basically what nm-connection-editor allows you to do. I also found somewhere that half-duplex is in the specification even for 1Gbps.

Looks like a complete fix to #413211. I've tested it on Arch Linux with different modes. For me Ignore, Auto and 10/100/1000 Mbps modes worked as intended as well as duplex setup for 10 and 100.