If you look closely, you'll see that corner isn't exactly rounded, and leaves some sort of background.
Problem happens on both X11 and Wayland.

System details:
Operating System: openSUSE Tumbleweed 20250509
KDE Plasma Version: 6.3.5
KDE Frameworks Version: 6.13.0
Qt Version: 6.9.0
Kernel Version: 6.14.5-1-default (64-bit)
Graphics Platform: X11