Comment 1 for bug 1468834

To me it looks like lightdm does the right thing in /usr/sbin/lightdm-session.

According to the files in /etc/X11/Xsession.d, which belong to x11-common, the intended behavior seems to be to not source ~/.xsession. Which standard protocol are you referring to? Are you possibly mixing it up with ~/.xsessionrc, which is sourced by lightdm if it exists?