I really don't know if this is a solution, but due to circumstances I've managed to get rid of this bug: I've made an upgrade on my son's PC and I've installed my older motherboard (if I can say so, because I bought it a couple of month before) a Gigabyte AB350 Gaming 3, on his PC, as it seemed more appropriate for his needs, and hopefully I'll solve my problem with FreeSync and/or AOC monitor. The new MB is a ASRock Fatal1ty B450 Gaming K4, main difference (for the topic) is that it has a DisplayPort graphics output. I've updated the graphic drivers to 18.12.2 (I doubt that had anything to to with our common bug), and the bug just vanished; I tried 3DMark and a few games with FreeSync enabled, and it all worked just fine. So, in conclusion: I would try to connect to a DisplayPort adapter (as the DP-HDMI cables seem to work only if you have a DP output on your PC), or change the MB, but I repeat: it worked for me, but I'm not 100% sure that this was the cause... Hope it helps!