This is an issue that happens with anything that uses KWin's blur but is more noticeable with window decorations that use rounded corners, such as Aurorae or Breeze forks like Sierra Breeze, Breeze Enhanced and Breezeway. If the corner radius is bigger than normal Breeze, the blur behind the titlebar still takes the shape for normal Breeze, with a sharp corner. This can be fixed by allowing the blur to take shape of the corner or give an option to disable blur for window decorations.
STEPS TO REPRODUCE
1. Use any window decoration with round corners
2. Enable blur effect on KWin effects
Sharp blur corner near round corners
Blur following round corners
Linux/KDE Plasma: openSUSE Krypton
KDE Plasma Version: 5.18.80 20200829T074113
KDE Frameworks Version: 5.73.0 20200817T080058
Qt Version: 5.15.0-ku.4.1
Requires additions to the libdecorations2 library and porting decorations.
*** This bug has been marked as a duplicate of bug 395725 ***