Mac screen stays lit on login prompt My 2013 iMac running Mojave 10.14.3 has had this problem for a number of years and spans a few OS upgrades. When in locked mode, the screen can stay lit indefinitely, until I hit Esc. 

Behavior is like this. I lock my Mac, or just walk away from it until it locks itself up. I touch my touchpad or press any key to light up the screen. The screen will the stay lit - for ever. I have to manually hit escape to put it back to sleep.

My settings:
I have screen saver on (10 minutes). 
In Power Savings, I have standard options. These are checked:
 Put hard disks to sleep
 Enable Power Naps

What can I do to fix this?
I was able to temporarily correct this problem by deleting: 

/Users/xxx/Library/Preferences/com.apple.screensaver.plist


and restarting my computer.

This worked for a little bit, but stoped once I connected my iMac to my MBP as a Target Display.