SUMMARY
This bug is related to the color scheme option "Tint all colors with accent color" and using accent color from current wallpaper (a very nice feature, btw).

Panels and windows constantly flicker. This is the same kind of flicker that happens when changing the accent color, except that instead of a single flicker, it flickers continuously.

This is a very finicky bug that happens only with a specific color scheme, wallpaper, and even display settings (details below).

STEPS TO REPRODUCE
1. Set display resolution to 3440x1440.
2. Make sure the display is either the only display or is set as the primary display.
3. Create a new color scheme based on Breeze Light, but with the option "Tint all colors with accent color" checked.
4. Apply the new color scheme.
5. Select and apply the option "Use accent color: From current wallpaper".
6. Set the display's wallpaper to Fallen Leaf by Martin Klapetek (shipped with Plasma).
7. Make sure wallpaper Positioning is set to "Scaled and Cropped".

Note: the order of the steps is not important.

OBSERVED RESULT
The panel and windows start to flicker continuously, as if constantly reapplying the color from the wallpaper.

EXPECTED RESULT
The panel and windows should flicker only once as the color from the wallpaper gets applied.

SOFTWARE/OS VERSIONS
KDE Plasma Version: 5.27.1
KDE Frameworks Version: 5.115.0
Qt Version: 5.15.13
Graphics Platform: X11

ADDITIONAL INFORMATION
3440x1440 is the native resolution of my external display. If I use any other resolution (larger or smaller, even with the same aspect ratio), the bug isn't triggered. Refresh rate doesn't seem to matter. The bug also doesn't trigger when my laptop's built-in screen is set as the primary one, regardless of the resolution set there (it doesn't support 3440x1440, though).

The accent color is picked from the primary monitor's wallpaper, and it seems that if that wallpaper is Fallen Leaf scaled and cropped to 3440x1440, the bug triggers.