When I use multiple monitors I get a range of odd behaviours, including a white screen, lock ups, failure to display anything on second screen. I've unplugged the second screen for now and all is OK except that adaptive sync does not work properly.
Sounds like a driver problem. What kernel are you using, and have you tried running Mesa-git?
When I set adaptive sync to "Always" in the settings the screen sort of flickers when I move the mouse. To be more precise the screen gets a bit brighter when the mouse is moved, then returns to previous slightly dimmer brightness when the mouse is stopped. There are no errors that I've found.
You can't find any errors because there are none. The brightness changing with the refresh rate is sadly how most monitors work today, and can't really be fixed. It's the whole reason for why adaptive sync is not set to always by default.
Why doesn't automatic work for you though?