Solution: Active apps and windows move from second monitor to primary monitor after the displays went to sleep

I have a dual monitor setup (Mac Mini + two Dell monitors). The monitors are set to sleep after a certain period of inactivity, or I can lock the screens and walk away. When I return and log back in to the computer, all active windows and apps have moved from the second monitor (typically on the right) to the main monitor (typically on the left). I wanted each monitor to keep the apps and windows exactly where I left them before the displays went to sleep. Here's a solution that worked for me:

1. Click on the Apple logo in the upper left corner of the screen
2. Click on "System Preferences"
3. Click on "Mission Control"
4. Make sure to select only "Displays have separate Spaces." If any other option has a check mark, please unselect it. See screenshot below:

Please let the community know if this solved the problem for you too.