Plugged laptop into my USB-C dock as I often do, but the two external screens were mirrored. I tried everything - replugging, rebooting, downgrading the kernel. The OS was only seeing one screen. Tried replugging everything into the dock. Finally I unplugged absolutely everything, even from the wall, replugged it and it worked perfectly. So I conclude it was electrical interference. Now I'm not so sure about USB-C "power going both ways".