Same problem here with Ryzen 3800x asus x570 chipset rx5700xt and samsung lc49hf90....
no flickering in 60hz but 100,120 and 144hz make flikering
the flikering same provide of the fps fluctuation see in the panel of the Samsung LC49HG90 IN GAME
no problem with 144hz on windows desktop
My old 1060 had no problem freesyncing this monitor without flicker, but as soon as I tried two different 5700 and an XT the flicker never left, it's present in any game and any video content, the only solution is to disable Freesync and keep enhanced sync, which is a downgrade.
We all know our monitors are perfectly fine, but the drivers are immature and we would like an official response.
I now have Nvidia 2070 Super, and of course the flickering is much less, but it seems a bit more than with 1060 6GB which is weird. But found that when I tune some game settings it's either not visible or minimal. Only in menus(often) or when FPS drops to 50 or below . For example in Assetto Corsa - it flickered every second no matter what settings, but I when I set in-game option: Render framers ahead (or something like that) to 1, it got rid of the flickering. It's important to have V-Sync on, but I found in Project Cars 2 I had to use in-game v-sync ON and not in Nvidia control panel(rather: application controlled) . It already had Render frames ahead set to 1.
So you might experiment with 5700. I remember some games were not that bad i.e. rFactor2.
I also experience flickering with Freesync enabled in at least one game, Final Fantasy XIV (only game I've been playing lately without a framerate cap). This is with a new RX 5700 XT, connected via DisplayPort to my Viotek GN27DB. When FreeSync is disabled, the issue goes away.
I actually thought there was a problem with the backlights at first, glad that's not the case and that I'm not the only one with this issue.