https://bugs.freedesktop.org/show_bug.cgi?id=102300
--- Comment #12 from fayn@mt2015.com --- Arch stock kernel 4.12.13-1 still boots one monitor black, but can be restored with xrandr.
Wayland sessions work, and they never lose signal in any situation. Xorg sessions do randomly cause black screen (https://bugs.freedesktop.org/show_bug.cgi?id=102956).