Xfwm4 does not center transient dialogs above their parent window for GTK4 apps
You can check this with e.g. gnome-text-editor, but any GTK4 app should do (typically with the about dialog). It works fine on Wayland and also on X11 with openbox, not sure if this is really specific to xfwm4 though.
Probably related: https://gitlab.gnome.org/GNOME/gtk/-/issues/4090