I cleaned this flex with 65% ethanol (didn't have 91% isopropyl) and the glitch has gone. But then I the attached the back lid and the glitch is back. Then I removed the lid, not even turning off the tablet, and the glitch went away again.
So there is a glitch with the lid on and no glitch with the back lid removed. It's incredible! What could be wrong? It functioned well for a decade with this lid on.
Under the spoiler are pics of the back lid from the internet, mine is exactly the same.
Maybe the element on the second photo got magnetized from electromagnetism from appliances in my kitchen and that's the problem?