Same situation on my HTPC : Automatic login, local account, "Login" screen configured for solid color background (configured with Settings), but "Beach" picture background forced on "Welcome" screen at PC startup (boot) or restart.
Finally, this registry tweak fixed it for me :
1. Run regedit.
2. In Registry Editor, go to following key:
3. In right-side pane, create new DWORD DisableLogonBackgroundImage and set its value to 1.
4. Restart your computer and it'll remove the background image from login screen.
(If you want to restore Login Screen background image in future, delete the DWORD DisableLogonBackgroundImage).
Result : now solid color background on "Welcome" screen at PC startup or restart, as I wanted.
Maybe you should try this out. Hopefully it works also for you (at least until the next Windows 10 "Feature" update ...).
You can find more information on this regedit tweak in https://www.tenforums.com/tutorials/9...dows-10-a.html How to Enable or Disable Sign-in Screen Background Image in Windows 10 (and also Google for "Disable Login Screen Background Image" to bring up additional info).
This registry tweak worked for me on a PC with Windows 10 Pro 64bit - version 1607 - OS build 14393.321.
Additional info : in my Settings > Personalisation > Lockscreen. I have the Lock Screen set to 'Picture', and 'Show lock screen background picture on sign-in screen' set to 'Off', but that did not solve the forced "Beach" picture thing.