Mac Mini, Samsung DisplayPort not sleeping.

I’m having some strange issues with my Mac Mini 2018 running a Samsung Ls34j550 monitor via DisplayPort. ( I also have a second different monitor running via HDMI behaving as expected.)

When the Mac Mini sleeps the Samsung display goes into a loop of no signal, saying it will go into stand by mode in 3 seconds, turning off then back on and repeating. Strangely when this happens an external hard spins up before shutting down and a USB powered device also powers on then off. Eventually the screen turns completely off and won’t turn back on unless the power is unplugged and re plugged. During this cycle the 2nd monitor remains in stand by.

I’d appreciate any advice in troubleshooting this.