Following - I have the same issue with the upper 5 keys on my ESQ-1. All research points to lack of connection to the keyboard controller IC or the IC itself is failing.
I’ve disassembled the keybed and cleaned all contacts and verified continuity with the traces to the header pins. I’ve cleaned and re-seated the keyboard to motherboard ribbon cable with no change. I’ve ordered a new keyboard ribbon cable to to rule that out. Following that I’ll pull the motherboard and reflow the header pins and IC socket pins.
If none of that works it’s likely a failing controller IC which is unfortunate as those are basically impossible to find on their own these days. I would have to live with the loss of the top five keys at that point (trigger via MIDI if I had to).
Keen to hear how you go with yours.