This is a GUI install.
I haven't noticed this on any non-GUI installs (e.g. Stretch Lite) but I usually run those headless and might not even notice.
Edit: This was on an installation that hadn't been updated in several months. A test case might be to install one of the older versions, disable autologin via `raspi-config` and then update to current.
Edit: I just reproduced it. I started with an installation that was Raspbian Jessie (not lite) last updated December 2017 and with the only user 'pi'.
- It did not autologin. I logged in but the desktop was unusable since no menu came up. Switch to a text console and run `raspi-config` to set desktop to boot to GUI without autologin.
- Ran 'apt update' and 'apt upgrade'. I ran this from a text console since the 'pi' desktop was messed up.
- Following reboot, desktop did not autologin.
- Logged in to text console as 'pi' and added user 'hbarta.' Rebooted to get back to login screen.
- Logged in as `hbarta'
- Edited sources.list and sources.list.d/raspi.list to use Stretch repos.
- Ran `apt update` and `apt upgrade` and `apt dist-upgrade` and rebooted.
- Following reboot system came up and logged into GUI as user 'pi'
At this point I do not know if it was the update alone or the update as user 'hbarta' that resulted in the return of autologin.
I'll test again with Stretch (2017-09-07) to see if it does it when logged in as 'pi.'
Test complete with Stretch 2017-09-07 -> current.
- Installed 2017-09-07 version of Raspbian Stretch
- Use `raspi-config` to set boot option to "Desktop"
- Reboot and confirm boot to display manager (not logged in.)
- Login and 'apt update', 'apt upgrade' and 'apt dist-upgrade.'
- Confirm that user 'pi' is automatically logged in to a graphical desktop.
In other words, it had nothing to do with performing the update logged in as 'hbarta' (and sudo to root.)
Edit.2 I just repeated the second test (update an earlier version of Stretch to current.) I recalled being asked about whether to update or preserve a lot of configuration files. My tendency in these cases is to go with the new configuration file unless I recall having modified the old one. I recall one of these was for the login screen (lightdm.) It occurred to me that selecting the new one might revert some setting back to the default (autologin 'pi.') However this upgrade asked no questions. It was the upgrade from Jessie to Stretch that involved a lot of config file updates. Optional config file update is not an issue with this behavior.