Nate Graham said (comment), that the VERY OLD BUG, has been resolved in 5.27, but unfortunetly IT HASN'T (I still have it in 5.27.9 and people have it even in KDE 6), I put this bug up again, because its still present in 5.27.9.

Still UNRESOLVED in 5.27.9

WHAT HAPPENS:
Just after system startup, when you had configured the panel to be opaque (should be the default) the "Application Lanucher"  is ugly translucent. It looks rediculous and ugly as hell.  I'm sorry, but I cannot stand it. Its like a splinter in they eye. You know its there.

STEPS TO REPRODUCE:
1. Set panel to opaque
2. Restart your system
3. Click "Application Launcher" = Its ugly translucent (and bad for eyes and contrast, visually ugly with wallpaper shining beneath...ouch!).

Moreover:
Application Laucher does not have separate settings for opaquenss, so it must follow the panel setup. But it has mind of its own and is ugly translucent. Fonts and icons look ugly. Not contrasty. Color looks ugly and washed out gray with a hint of bright color (magic hour photo!), i.e. wallpaper shines through. This is so ugly.

$ kinfo
Arch Linux (Not Manjaro)
KDE Plasma Version: 5.27.9
KDE Frameworks Version: 5.112.0
Qt Version: 5.15.11