Gigabyte Z170X-UD5 - restart hang at power cycle (shutdown works fine)

• Z170x-UD5 (bios F23i and F23f tested)

• 6700k

• Asus ROG-STRIX-RX580-O8G-GAMING

• Vanilla install

• 10.14.6

• Using iMac17,1 and headless iGPU enabled.

I'm having a bit of a head scratch as to why my system doesn't want to restart anymore. It hangs after the apple shutdown circle and after the monitor goes blank (where the actual restart POST occurs).

The system will reboot and then hang, and you can hear the fans start spinning more.

Nothing I can really tell from console or verbose logging.

Everything works fine when using "shut down". This system has been in use since 2016.

I don't recall it hanging when I had 10.13.x and an Nvidia 980ti, (but I don't restart much). I noticed after I removed the 980ti and replaced it with a 580 AMD card, (and since updated to 10.14.6).