When I right click in menu and choose Forget All Documents or Forget All Applications nothing happens. It just blinks a couple of times and all my recent apps and docs reappearing again. Before that my apps and docs randomly flushed from time to time. This happened on plasma 5.12.2 and 5.12.3.
cannot confirm on Arch Linux, plasma 5.12.3. I can clean recent documents and recent apps lists.
This sometimes is related to a damaged kactivitymanager database. Please run kactivitymanagerd stop mv .local/share/kactivitymanagerd/resources .local/share/kactivitymanagerd/resources-old restart kactivitymanagerd and check if it fixes it for you.
Possibly related issue https://bugs.kde.org/show_bug.cgi?id=376814
The feature is implemented, but it sometimes doesn't work due to a corrupted kactivities database. *** This bug has been marked as a duplicate of bug 376814 ***