If I understand you correctly, your screensaver starts up when the system is idle for your specific time but then the normal Desktop reappears without any obvious activity causing that.
It's possible that there is some task running in the background that causes this behavior.
- Does the issue happen if you boot up the computer and do nothing until the screensaver activates?
- Do you have a wireless mouse or wireless keyboard? It's possible that there is some wifi interference signal that's being picked up and causing the screensaver to stop
- Try starting the computer in a "clean boot" configuration (see below). If the problem does not happen in this configuration, then it's pretty likely that the cause is some application or service that normally starts when Windows starts. You'll have to re-enable your startup items in a logical fashion in order to pinpoint the culprit.
How to start in a clean boot configuration:
https://helpdeskgeek.com/windows-7/perform-a-clean-boot-in-windows-7/
Remember: the clean boot configuration is only a diagnostic tool; it is not a fix for anything.
Also, although it's not responsive to your question, consider that screensavers are generally no longer needed now that hardly anyone uses CRT displays any more. See http://www.thewindowsclub.com/are-screensavers-necessary-still-needed You can use the "slideshow" setting to rotate pictures on your Desktop background to achieve a similar effect (although, of course, the computer is not locked if the Desktop is displaying).