Launching fuzzel 1.10.1 (or 1.10.0) on KWin 6.0.3 with an --anchor=center flag or without any flags makes the window pop up in the top left corner of the screen. Every other --anchor value works as expected, .e.g. with --anchor=bottom the window pops up in the bottom-center of the screen.
I'm not sure if this is a KWin bug or a fuzzel bug, because on Sway --anchor=center works as expected, but on the other hand fuzzel 1.9 opens in the center on KWin, so the compositor can do it.
Steps to reproduce:
Launch fuzzel --anchor=center on KWin
What hapens:
fuzzel window pops up in the top left corner of the screen
What I expected:
fuzzel window pops up in the center of the screen