Same exact problem here. In fact, have three fairly large issues with my install of Cinnamon, all related to the desktop and/or login.
1-Cannot lock screen. Keystrokes nor menu selection will lock it. Nothing happens at all. And, yes, I totally need this functionality.
2--Time. I have the time configured correctly and when I log in, it grabs the network time and the world is a happy place. Unfortunately, from that point on, it never UPDATES the time.
3--I'm running a USB stick and booting my Lenovo laptop to it. Again, works well. But I cannot for the life of me, get MY user ID to appear first; it always boots into Minut Live User.
I think Cinnamon has great, seriously great potential. But if it was as easy-going as plain jane Debian, I'd be a happy camper.