This issue is with the Light Explorer window, and more specifically its search bar. The issue identified is that whenever the results of the search change, the search bar will lose focus and additional characters won't be added to it.
# open the attached repro project
# add a bunch of point lights and directional lights to the scene
# open light explorer
# attempt to type "point" in search bar, results in "poi" being entered and an unfocused search bar
# notice that suddenly the search bar loses focus and following key presses are ignored

Fix found by [~mads], [Github Commit|https://github.cds.internal.unity3d.com/unity/unity/commit/7778ad78eb8de86618380ab2716c3c4c3e8f37c5]