EDIT: Manjaro repos have updated qt5-wayland 5.15.5+kde+r38-3 which does not need workaround below.
All KDE Wayland users, as a workaround for “top left corner” issue you can create file /etc/profile.d/qt5-wayland-regression.sh with following contents
export QT_WAYLAND_DISABLE_FIXED_POSITIONS=1
The issue was caused by this commit which was reverted 4 hours ago. I found the workaround in this commit which caused the issue.