This is due to a feature that rolled out with KM10, in that when the cursor is in the KM Editor search field, tapping a modifier key inputs that key character.
I brought this up shortly after v10 rolled out a couple of years ago (see my linked post below), asking if there was a way to disable it, to which @peternlewis replied that there wasn’t, but if it was sufficiently annoying to enough people he would perhaps change the behavior (or something to that effect). I haven’t really seen many people bring it up in the two years since, and I myself have simply gotten used to it, so I doubt the behavior will be changed in the future.