@Mitsuya: I don't know systemd well enough to tell. What I did before drawing the conclusion that it's somehow gdm3 related was to install and test with lightdm. With lightdm the im-config variables are preserved also on GNOME shell (with or without Wayland).
@Mitsuya: I don't know systemd well enough to tell. What I did before drawing the conclusion that it's somehow gdm3 related was to install and test with lightdm. With lightdm the im-config variables are preserved also on GNOME shell (with or without Wayland).