The wallpaper keeps reverting back to the default Mojave wallpaper
I have my own wallpaper set in my Macbook. Recently when I boot my Macbook, the welcome screen utilizes the default wallpaper of Mojave (the desert photo) instead of my wallpaper. I was bothered that I searched this problem prior to this post. I found a post where I should just uncheck "Displays have separate spaces" in Mission Control. It worked for a while but after a day of not booting, still the same problem.
Any suggestions or fixes? Or should I update to Catalina? I have recently installed the latest Security Update for Mojave.
MacBook Air 13", macOS 10.14