In viewing your video, YouTube gave me this one as a suggestion: https://youtu.be/1Wzf4V1KBug
Looks like it may be the answer, and even if not, should give you an insight into how to open your keyboard and how it works. From what I can make out, beneath each key are 2 switches, each consisting of a set of 2 interwoven tracks. Pressing a key presses a conductive rubber pad onto the tracks, linking them and closing the switch. One of the 2 switches is arranged to close a little earlier in the downward travel of the key than the other, and the electronics uses the time difference to deduce how hard you struck the key.
If I heard him right, the guy in that video used spit to clean the contacts! Not sure I'd agree with that. Get some isopropyl alcohol. The conductive rubber may equally be to blame. But before trying to clean that, clean the ones on the very lowest or very highest key and make sure you don't make them worse. It could just be a layer of graphite on the rubber which might rub off with cleaning.