I am running Gentoo with plasma 6.4.5 and have had a strange issue for the past couple revisions where every so often the clipboard will get stuck and not update the contents.
When it happens I know it has happened because the clipboard indicator will be present on the system tray area. It will have a single item in the history even when the history is set to 20 items (the default). I can highlight and middle click paste within the same window, but as soon as I try to middle click paste in another window, it will paste the contents of the clipboard. If I try to copy something else, the contents of the clipboard will not change until I manually clear the clipboard history. Then it will work fine for a while longer.
I don’t know how to manually trigger the issue and the only indicator I get when I clear the clipboard contents is: qrc:/qt/qml/org/kde/plasma/private/clipboard/ClipboardItemDelegate.qml:121:32: Unable to assign [undefined] to double