retitle 56661 SDDM does not do the equivalent of 'login shell --login', 
unlike other login managers
thanks

A digged a little, and found  that:

For GDM, things work, for SDDM, they don't.

There is a 'xinitrc' procedure in gnu/services/xorg.scm that generates a 
configuration file that has a fallback .xsession that does a --login. 
This is used by gdm-wayland-session-wrapper, <gdm-configuration> and 
slim-shepherd-service.  However, nothing similar appears to be done for 
SDDM.  So it appears that the SDDM service needs to be tweaked to use 
xinitrc or such.

Greetings,
Maxime