It looks like some update on September 30 or October 1 inhibits correct tracking of GUI logins. At a guess, I think lightDM is not updating /var/log/wtmp. I'm not sure, though, and I'd appreciate suggestions on how to narrow it down so I can make a proper bug report.
I often log in to my desktop via ssh, then execute 'w' or 'who' to see if anyone is logged in, before I do any work on my system. I executed 'w', found that there was only one user logged in, which was me, then I started x11vnc and connected to my desktop, only to see that my stepson was logged in and playing Minecraft. I tried 'who', and it indicated only my login; 'last' didn't show my son's login today.
'last' did show his login via GUI from yesterday, before I applied a batch of updates, so that's why I think this was just broken:
I did see that his processes were correctly listed by 'ps' and 'top'.
kidongui tty7 :0 Thu Sep 1 16:50 - 21:37 (04:46)