I'm not sure it's Falkon or QtWebEngine issue, but with newer Mesa 21.3.x webengine view has some blinking, no issue on any other components. I just update, staying to 21.2 till now.

Mesa 21.3.4 is in the build repo since 2 days, tested if it includes the above patch (from 3 days ago? (or, which mesa are you reporting).

The patch is not officially merged yet. It could be part of 21.3.x where x >= 5.

10 days later

Patched Mesa is now up in the build repo, please report if that fixes your issues (I don't see it, so can't check).
I had hoped the patch would be merged before applying, but the MR keeps showing as blocked, no indication why, looks like it won't be part of Mesa 21.3.5. So this patched version won't move until it is shown it is an improvement.

I applied by myself, since using only Falkon, it was unusable. No issues since then, thanks for the fix.