“We have been tracking an issue that has been reported for HDMI displays (mainly, TVs used as a secondary display) that doesn’t seem happen all the time. These are color artifacts and not “squiggly lines”,” Michael Herf from F.lux told us.
“Anytime there are a lot of changes in video drivers, some problems happen like these, and usually they are fixed quickly.”
The author of the app also confirmed that F.lux is working properly on single-display systems.
If you see squiggly lines or artifacts after upgrading to Windows 10 version 2004, you can try to update your GPU drivers, disable F.lux temporarily or you can also roll back to the previous version of Windows.