Hi,
I'd like to say that I *just* had this problem on Driver Version 19.7.2 with a Fiji series GPU over HDMI.
The fix was a driver update. I suspect a driver rollback would have done the same.
If you are curious, yes: FreeSync stability, input lag reduction, and screen flicker effects vary wildly with driver versions. I've had problems with bad driver versions, but never *that* bad.
These latest driver versions also do not work with the (leaked) AMD Windmill demo, as the demo now fails to clamp the FPS and will run at insanal rates.
Oddly, NVIDIA's G-Sync demo works fine, you just can't toggle FreeSync on/off in the menu. It still demonstrates its functionality, however.
I realize this reply is months late to help you (or you already updated your driver and that may have helped), but this shows up on Google, so maybe it will help someone.