SUMMARY
After installing almost any KDE Plasma distro, the Very First time you edit the settings on the Login Screen (SDDM) page (Settings > Colors&Themes > Login Screen), whatever change(s) you make are not applied. So, you must repeat your changes and the second time, it works. This applies both to changing the background theme of the Login Screen and also changing the "Automatically log in:" settings on the Behavior page. 

STEPS TO REPRODUCE
1. Install a KDE Plasma distro such as CachyOS  (this happens on many KDE Plasma distros)
2. After install, immediately go to Settings > Colors&Themes > Login Screen (SDDM)
3. Change the background theme or on the Behavior page, make a change to Automatically Log In
4. Click [Apply], enter your password and then click [OK].

OBSERVED RESULT
After you enter your password and click [OK], your changes are not accepted and the settings page your on reverts back to the previous settings. Then if you repeat the exact same change, click [Apply], enter your password and click [OK], the second time and forever after it will work perfectly. Your very first setting change for Login (SDDM) will not be applied regardless if your first setting change is to change the background theme or to make a change to Automatically Login on the Behavior page. All future changes work fine.  

EXPECTED RESULT
Obviously, it should work the very time you edit settings for the Login Screen (SDDM)

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
(available in the Info Center app, or by running `kinfo` in a terminal window)
Linux/KDE Plasma: CachyOS
KDE Plasma Version: 6.4.3
KDE Frameworks Version: 6.17.0
Qt Version: 6.9.1

ADDITIONAL INFORMATION
I LOVE KDE PLASMA !!!!!!