Hi,
I have 2 monitors with freesync. When I try to enable adative sync on sway, it makes most windows flickers. If I disable it on sway but keep it on in monitor settings, It gets a little bit darker and during game play it won't work because I can see tearings even though FPS is in the range of free sync.
Is your graphics card from Nvidia? Their 535 driver version introduced flickering issues of some sort or another for a lot of people. If I remember correctly you have two choices: One is to downgrade to version 525. The other is to disable G-sync/Freesync.
Nvidia is aware and is working on it. Doesn't mean that it isn't a pita, of course.
If you want to help you can go to https://github.com/NVIDIA/open-gpu-kernel-modules/issues/511 and use their debug script to give them more to work with. The issue is present in their regular driver and the open kernel modules one.