SUMMARY In the translation file from kcm_keys, I found this string: Shortcut <shortcut>%1</shortcut> is already assigned to action <interface>%2</interface> of <application>%3</application>.<nl/><nl/>Reassign to action <interface>%4</interface> of <application>%4</application>? Note that %4 is used twice. I think one of them should be %5.
Fixed by https://invent.kde.org/plasma/plasma-desktop/-/commit/6370c61f6d704f4e85105b4111272692fc83d13a