I always keep my laptop connected to my external monitor, with its lid closed, and put it on Sleep when not in use. Lately, I've noticed that, when waking the PC up from sleep, the external monitor is no longer the primary display but an extended one. If I open the laptop lid and close it again, the external monitor will go back to being the primary display - but having to do this each time is becoming annoying.
I am using Windows 10 and the latest graphics card & monitor drivers.