The problem is that applications (in my experience it has only been browsers, both firefox and chromium) request that power management be suppressed, which overrides an automatic screen lock timeout configured through power management. On its own this behaviour makes sense, but it can result in a silent failure of the timed automatic screen locking, which is a significant vulnerability for users who are depending on the auto screen lock for the security of their session. 

This problem is exacerbated by poorly designed websites that get the browser to request power management suppression for reasons that are not obvious to the user (a background webRTC PeerConnection in chromium is the recent example I saw).

I would expect that there would be an externally visible notification of when this suppression of power management occurs, so that a user who is depending on screen locking will be aware that it has been disabled. Alternatively, there could also be a way of configuring power management to override the suppression requests for users who value screen locking (and other power management features) over the convenience of the automatic suppression. 

It is possible to click on the "Battery and Brightness" tab of the system tray to see a message about suppression of power management, but there is no externally visible notification when the suppression occurs. It is too tedious to periodically click into this area to check if there is a suppression. 

It is also possible to configure a button or keyboard shortcut for quick screen locking and do this manually just in case the automatic screen lock is being suppressed. This is the workaround I am currently using, but it is basically just a replacement for a timed screen lock that can't be trusted to work as configured. 

Reproducible: Always

Steps to Reproduce:
1. Use power management to configure a short timeout on the screen lock
2. Open a poorly designed website in chromium or firefox that makes background requests to suppress power management (some pages from http://www.laprensa.hn/ in chromium containing background webrtc peerconnections, for example). 
3. Get up and leave the computer with that tab open. See how long it takes for someone to realize you left your session open and start digging around in your stuff :p

Actual Results:  
The screen lock will not work because the power management is suppressed. 

Expected Results:  
Popped up a notification (or have the option to enable such a notification) as soon as the power management is suppressed so that the offending website tab could be closed, or the screen could be manually locked.