SUMMARY
On a multi-monitor setup, if you open System Settings → Wallpaper, change the folder list or check/uncheck an image, then click another display, changes are discarded without warning.

STEPS TO REPRODUCE
1. Open System Settings.
2. In the sidebar, click Wallpaper.
3. Add or remove a folder, or check or uncheck a wallpaper.
4. Click another display.

OBSERVED RESULT
Changes to the image list are lost silently, and the Apply button grays out. If you instead click another entry in the sidebar, the app warns about changes being discarded ("Apply Settings").

EXPECTED RESULT
The Wallpaper KCM should store a working copy of wallpaper folders/images for *each display* currently active, and save all changes when I press Apply.

If you decide to only save an image list for the currently selected display, then when the user switches displays, KDE should warn that changes will be lost, and offer to apply or discard these changes ("Apply Settings").

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 6.2.5
KDE Frameworks Version: 6.10.0
Qt Version: 6.8.2
Kernel Version: 6.13.1-arch2-1 (64-bit)
Graphics Platform: X11
Processors: 12 × AMD Ryzen 5 5600X 6-Core Processor
Memory: 15.5 GiB of RAM
Graphics Processor: Mesa Intel® Graphics
Manufacturer: Gigabyte Technology Co., Ltd.
Product Name: B550M DS3H

ADDITIONAL INFORMATION