After waking the system from suspend to RAM (Suspend function in KDE menu) the session (sometimes) is not immediately locked. It displays the user session for couple of seconds. It maybe depends from what the system load is. In any case the session should be locked before anything else. This is a thing since KDE 4 days. Still present. https://forum.kde.org/viewtopic.php?f=225&t=110378 ArchLinux 32-bit.
I've also observed this on Ubuntu Xenial (64-bit). Very infrequently, so I agree system load may be related.
I have seen this happening just a day before.
*** This bug has been marked as a duplicate of bug 316734 ***