The current pre-release of the client ("pioneer" in the version) is only compatible to itself. So you can only play with other testers. Please be aware!
I have the same issue in vanilla FA and FAF. When the 2nd monitor is enabled the game flickers on both monitors, when only 1 is enabled it works flawlessly.
Been playing FA weekly on this configiration for the last 5 years, unsure what caused the change, its sucks though