Im having similar issue with my HS setup, dell latitute 3480, i5-7200U
Once the hdmi plugged in, the laptop display went blank, only could see the external tv
But, when i put my laptop to sleep & wake up again, both screen got display (hdmi still connected)
Even the hdmi could be plugged off & in (after sleep), the laptop display is fine
What could be the culprit?
Dell’s MacBook Pro IORegistry.zip