SUMMARY
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Within the desktop settings, the setting "Previews" under the section "Icons" only works in part. It is possible to show or hide all previews, but unchecking some preview plugins within the "Configure Preview Plugins" option yields no result and all plugins are reset to active once the changes are applied. 
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STEPS TO REPRODUCE
1. Have a non-empty folder and a file on the desktop (Folder View mode)
2. Open the "Configure Desktop and Wallpaper" settings by right-clicking on the desktop and go to the "Icons" section
3. At the bottom of the page, open the "Configure Preview Plugins" menu
4. Uncheck the "Folders" plugin and apply all changes.

OBSERVED RESULT

No change takes place: folder previews are still visible and reopening the "Configure Preview Plugins" menu shows that the change has not been saved.


EXPECTED RESULT

Folder previews disappear and the Folder plugin within "Configure Preview Plugins" remains unchecked.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma:  openSUSE Tumbleweed
KDE Plasma Version: 5.27.10
KDE Frameworks Version: 5.114.00
Qt Version: 5.15.12

ADDITIONAL INFORMATION
This also applies to the same setting within the Folder View plasmoid, but not to the Previews settings within Dolphin.