I have had lots of issues with the lock screen in MATE. CTRL+ALT+L doesn't seem to do anything most of the time. Selecting lock from the menu does nothing most of the time. And there are times where I shut the laptop lid and open it and there is no lockscreen I am just right into the system.
I did submit a bug report that popped up about it also I am seeing the following.
$ grep -i screensaver ~/.xsession-errors
mate-session[1560]: WARNING: Detected that screensaver has left the bus
** Message: 23:43:32.787: Screensaver is not running!
** Message: 23:43:48.744: Screensaver is not running!
** Message: 23:44:14.778: Screensaver is not running!
** Message: 23:44:17.618: Screensaver is not running!
mate-session[1560]: WARNING: Detected that screensaver has left the bus
** Message: 11:08:58.813: Screensaver is not running!
** Message: 11:09:05.115: Screensaver is not running!
** Message: 11:09:14.960: Screensaver is not running!
** Message: 11:09:28.550: Screensaver is not running!
** Message: 11:09:28.769: Screensaver is not running!
** Message: 11:09:28.967: Screensaver is not running!
** Message: 11:09:29.125: Screensaver is not running!
** Message: 11:09:29.285: Screensaver is not running!
** Message: 11:09:31.808: Screensaver is not running!
** Message: 11:09:31.989: Screensaver is not running!
** Message: 11:09:32.124: Screensaver is not running!
** Message: 11:19:53.665: Screensaver is not running!
any ideas how to work around this?