You didn't post more info about your system, but I'm guessing you have gnome or budgie? These two experienced similar problems with DM's with this update. There are several similar threads about this on the forum with solutions, depends of your situation, gnome / budgie users needed to install and enable GDM:, Xfce users Lightdm gtk greeter ...
sudo systemctl enable gdm.service -f
Check few latest forum posts for 'black screen after update'
I don't know how to set auto login. But you could try something else. LightDM is default for Xfce, correct? You could try following steps, disable MDM from starting at login then install LightDM and greeter and enable it. But before you do this, this time there may be one additional step, (downgrading webkit2gtk) that may be the culprit of your original problem.
I understand. The reason I mentioned LightDM is because it is default DM for Xfce. (or it used to be?) But whichever one you like and works better for you, of course. I switched once to MDM and come back to LightDM.