SPCR

I recently installed a 460W Seasonic Platinum Fanless PSU and moved my system to a new Antec Solo II.

I'd only switched the PC on and off a few times before the other night after shutdown the case power light stayed on. I restarted the PC and this time when I shut down the PC at the end of the process the rear fan suddenly ramped up to 100% and the power light stayed on. Power draw was 20W according to my power monitor.

Has anyone seen this before?

Fan would be a mobo issue - not the PSU. If you haven't already, do a hard reset of your UEFI/BIOS and see if the problem goes away.