Execute the following to re-apply HKCU registry keys after changes in a logged on session:

RUNDLL32 USER32.DLL,UpdatePerUserSystemParameters ,1 ,True

This may need to be run twice with a 2-3 seconds in between.

Sources:

• edugeek - Forums - Technical - Windows - Force Registry Refresh
• Rob van der Woude's Scripting Pages - RUNDLL and RUNDLL32 #45 Activate registry changes

Alternate ways:

• Griffon's IT Library - Windows 10 – Refreshing Desktop wallpaper without logoff/logon (PowerShell script – Ver. 2.0)
• Griffon's IT Library - Windows 10 – How to Force Refresh of Desktop Wallpaper (without logoff/logon)
• Helge Klein - Free Tool: Refresh the Desktop Programmatically
• Phil's Tech Blog - Wallpaper Changer Command Line Utility, see also GitHub - philhansen/WallpaperChanger.
• SG20 - Wallpaper Changer Command Line Utility by Phil Hansen.