I have the same config (Sapphire Vega 64 + Asus xg32vq) and also Problems with Freesync. As soon as i start a Game with Freesync enabled, i get Black Screens. Without Freesync everything works fine. Tried 3 different DP Cables but nothing works. Same Problem with the newes Beta Drivers.
The 19.1.1 optional drivers appear to have a fix for this issue. I haven't tried them yet, will wait for them to become official before I do so.
However i'll be curious to hear from anyone else if they give them a whirl?
Did you clean install the driver. I find the when freesync flakes out it is usually because I did the express install. Try a clean install or DDU.