Initial Summary:
Slowroll - XDM doesn't launch default Plasma X11 session

To reproduce:
1-install minimal X role
2-install patterns-kde-kde_plasma constrained by solver.onlyRequires = true
3-ensure no DM is installed except XDM
4-ensure plasma5.desktop is the default xsession
:
# inxi -S
System:
  Host: ara88 Kernel: 6.4.12-1-default arch: x86_64 bits: 64
    Console: pty pts/0 Distro: openSUSE Tumbleweed-Slowroll 20231001
# update-alternatives --display default-displaymanager
default-displaymanager - manual mode
  link best version is /usr/lib/X11/displaymanagers/xdm
  link currently points to /usr/lib/X11/displaymanagers/xdm
  link default-displaymanager is
/usr/lib/X11/displaymanagers/default-displaymanager
/usr/lib/X11/displaymanagers/console - priority 5
/usr/lib/X11/displaymanagers/xdm - priority 10
# update-alternatives --display default-xsession.desktop
default-xsession.desktop - manual mode
  link best version is /usr/share/xsessions/plasma5.desktop
  link currently points to /usr/share/xsessions/plasma5.desktop
  link default-xsession.desktop is /usr/share/xsessions/default.desktop
/usr/share/xsessions/icewm-session.desktop - priority 15
/usr/share/xsessions/plasma5.desktop - priority 25
#

Actual behavior:
1-yellow-gray display screens with a decoration-free Xterm launch
2-WINDOWMANAGER=startplasma-x11 startx in the Xterm launches an apparently
normal Plasma session

Expected behavior:
1-Plasma session launches directly

Additional information:
1-This is not unique to Slowroll, or this particular PC. I have it happening in
Leap installations as well. This particular Slowroll was was born as a zypper
dup from a cloned 15.5, which has the same problem.
2-This is not a recent development. ISTR 15.4 did it too, at least 6 months
ago, but rather than reporting I installed KDM3 or TDM on them instead of
keeping XDM only.