I'll post my specs again since I have the same issue, and have/had the same issue since my Windows10 version 2004 update back in August, and I would like this issue fixed.
My current build:
OS: Win 10 Pro version 2004
OS Build: 19041.508
Monitor 1: 3840x2160 (scale 250%) (This one has the flickering)
Monitor 2: 1920x1080 (scale 100%) (This one does not flicker)