Back when i ran a 1080p freesync monitor with a RX 580 i had some freesync issues where it would only work on a older driver as well. There was plenty others with other freesync monitor on both HDMI and DP connection that didnt have an issue even of the newest drivers. My conclusion is that some of these freesync screens are just not behaving well. I would try and look at CRU and see if you can mess with that to get it to behave. But i fully understand how frustrating it is. I was furious back then