Dual screen bug in Sierra 10.12.1?


I have an electron app that is designed to launch on two screens.

It is working fine on El Capitan and on Sierra 12.0. But after updating to 12.1 there is a problem. The first screen launches fine, but the second screen goes white and then the white area scrolls off to the left, and closes down, accompanied by a beep sound. So only one screen is launched.

I reinstalled El Capitan on same machine and it works fine again, so it definitely seems to be an issue peculiar to Sierra 12.1.

Does anyone know why this is happening or know a way to fix?