MINIMAL STEPS TO REPRODUCE
1. Edit /usr/share/xsessions/plasma5.desktop and change the Exec= line to
Exec=konsole
2. Remove the TryExec= line
3. Log out and select that session in SDDM or LightDM or any other login screen
and log in


EXPECTED BEHAVIOR
Konsole is launched and no Plasma session is started


ACTUAL BEHAVIOR
Plasma starts normally


REPRODUCIBILITY
This happens for any X11 session listed in /usr/share/xsessions/. It does NOT
happen for Wayland sessions listed in /usr/share/wayland-sessions/.


ADDITIONAL INFORMATION
This was discovered while debugging why KDE's Plasma dev session instructions
(https://community.kde.org/Get_Involved/development#Plasma) were not working.
The instructions direct the user to create new sessions in
/usr/share/xsessions/ and /usr/share/wayland-sessions/ with customized Exec=
keys that launch a special script instead of the default thing. This works for
the Wayland session but not for the X11 session, because the command specified
in the Exec= key appears to be ignored or overridden and replaced with some
default "start plasma" script.