Macbook Pro mid 2012 non-retina dual gpu built-in screen turns off when I connect an external projector

Used to work perfectly with Sierra, but the issue is suddenly present with High Sierra.

Tried to reboot while projector connected and even did a nvram reset - none of that helped. I only see desktop on the projector and built-in screen turns off. App store didn’t give me any firmware updates too when I had the projector connected. I could only use both projector and built-in screen simultaneously when they were mirrored. Might be something to do with dual graphics switching? And of course I tried brightness buttons.