Screen lock shuts down all applications in 17.10

I use a 5 minute screen blank on 16.04, 17.10 and 18.04 beta, and it works as expected for me. You might also want to check launchpad bug reports, but I suspect that something else is causing problems with your system than the lock screen and blanking alone. For example, I wonder if some extension is crashing the gnome-shell (or something equiv if you are using a different desktop)?

You might check logs e.g. journalctl -b0 to look for suspicious errors in and around the time of the blank or the unlock. journalctl is better than dmesg for this because it provides human readable dates for logged events.