Thread Code61 - some keys play a semitone discord

My Code, bought used but near-new, has begun glitching... a small number of keys (currently Eb2 B2, G3, but that can change!) will play a semitone discord; themselves and the nearest note above them. Weirdly those notes are fine an octave higher but glitch again 2 octaves higher... (ie) the fault repeats every 2 octaves with those same 3 notes! The issue may resolve for a week and then reappear, sometimes with slightly different keys misbehaving.
I did suspect that recent humidity was a factor, and found a YouTube post where a different brand of keyboard was exhibiting the same behavior. The owner also implicated humidity as did many players who responded with similar stories. Gentle application of a hair dryer also *temporarily fixed my issue for a few hours. I have to say that neither of my other 2 keyboards have ever exhibited this problem. I also once lived near the equator for 10 years, owning several keyboards that never glitched like this.
I opened up the Code61 (very clean inside as expected of a newish board) and removed the bottom 5 keys to expose the key contact of the misbehaving lowest Eb. I removed the strip of rubber boots covering the key switches and gently cleaned the contacts with ISP alcohol, reassembled and... boing! That didn't work!
I'm tempted to completely remove the keybed and clean every contact, but that initial experiment doesn't give me confidence that it will be worth the effort Has anyone experienced similar?