X11 works from SDDM, Wayland does not but it should. This is not a Plasma issue.
galaxy/qt6gtk2 0.3-2 (artix-style) [installed]
GTK+2.0 integration plugins for Qt6
cat ~/.local/share/sddm/wayland-session.log
dbus-daemon[2527]: [session uid=1000 pid=2527] Activating service name='org.kde.KSplash' requested by ':1.0' (uid=1000 pid=2528 comm="/usr/bin/startplasma-wayland")
dbus-daemon[2527]: [session uid=1000 pid=2527] Activating service name='org.freedesktop.portal.Desktop' requested by ':1.4' (uid=1000 pid=2544 comm="/usr/bin/kcminit_startup")
dbus-daemon[2527]: [session uid=1000 pid=2527] Activating service name='org.freedesktop.portal.Documents' requested by ':1.5' (uid=1000 pid=2547 comm="/usr/lib/xdg-desktop-portal")
dbus-daemon[2527]: [session uid=1000 pid=2527] Activating service name='org.freedesktop.impl.portal.PermissionStore' requested by ':1.6' (uid=1000 pid=2553 comm="/usr/lib/xdg-document-portal")
dbus-daemon[2527]: [session uid=1000 pid=2527] Successfully activated service 'org.freedesktop.impl.portal.PermissionStore'
dbus-daemon[2527]: [session uid=1000 pid=2527] Successfully activated service 'org.freedesktop.portal.Documents'
dbus-daemon[2527]: [session uid=1000 pid=2527] Activating service name='org.freedesktop.impl.portal.desktop.kde' requested by ':1.5' (uid=1000 pid=2547 comm="/usr/lib/xdg-desktop-portal")
/SNIP/
(kwin_wayland:2594): Gtk-WARNING **: 12:26:05.734: cannot open display:
qt.qpa.wayland: "wl-shell" is a deprecated shell extension, prefer using "xdg-shell" if supported by the compositor by setting the environment variable QT_WAYLAND_SHELL_INTEGRATION
qt.qpa.wayland: "wl-shell" is a deprecated shell extension, prefer using "xdg-shell" if supported by the compositor by setting the environment variable QT_WAYLAND_SHELL_INTEGRATION
qt.qpa.wayland: Loading shell integration failed.
qt.qpa.wayland: Attempted to load the following shells QList("xdg-shell", "wl-shell", "ivi-shell", "qt-shell")
qt.qpa.wayland: Loading shell integration failed.
qt.qpa.wayland: Attempted to load the following shells QList("xdg-shell", "wl-shell", "ivi-shell", "qt-shell")
qt.qpa.wayland: "wl-shell" is a deprecated shell extension, prefer using "xdg-shell" if supported by the compositor by setting the environment variable QT_WAYLAND_SHELL_INTEGRATION
qt.qpa.wayland: Loading shell integration failed.
qt.qpa.wayland: Attempted to load the following shells QList("xdg-shell", "wl-shell", "ivi-shell", "qt-shell")
org.kde.startup: "kdeinit5_shutdown" QList() exited with code 255
startplasma-wayland: Shutting down...
startplasmacompositor: Shutting down...
startplasmacompositor: Done.
qt.qpa.plugin: Could not load the Qt platform plugin "wayland" in "" even though it was found.
qt.qpa.plugin: Could not load the Qt platform plugin "wayland" in "" even though it was found.
qt.qpa.xcb: could not connect to display :1
qt.qpa.plugin: From 6.5.0, xcb-cursor0 or libxcb-cursor0 is needed to load the Qt xcb platform plugin.
qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.
Available platform plugins are: eglfs, vkkhrdisplay, linuxfb, offscreen, wayland-egl, minimalegl, minimal, xcb, vnc, wayland.
qt.qpa.xcb: could not connect to display :1
qt.qpa.plugin: From 6.5.0, xcb-cursor0 or libxcb-cursor0 is needed to load the Qt xcb platform plugin.
qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.
Available platform plugins are: eglfs, vkkhrdisplay, linuxfb, offscreen, wayland-egl, minimalegl, minimal, xcb, vnc, wayland.
I did see