This might be a dupe of https://bugs.launchpad.net/ubuntu/+source/gnome-screensaver/+bug/1291365. Although in that case it's described as only happening after having left the desktop locked for a while, instead of immediately after locking.
This might be a dupe of https:/ /bugs.launchpad .net/ubuntu/ +source/ gnome-screensav er/+bug/ 1291365. Although in that case it's described as only happening after having left the desktop locked for a while, instead of immediately after locking.