When I connect an external monitor, the internal monitor goes black. I can use the command

sudo killall -HUP WindowServer

It "restarts" the GUI and throws me back to the login screen. It wakes the internal display and it works. It is a little troublesome to do this every time I connect an external display. Has anyone else encountered this issue and has a fix to suggest?

It is the same whether I use the HDMI port or USB-C and adapter.