Hmmm… Yes, that sort of behavior us usually associated with instability in the cap. touch keyboard. Sometimes, here in the Lab, a synth will sound just because it’s humid. Does this happen consistently, all the time, or is it pretty erratic? Try placing a finger on the edge of the PCB, on the blue border but beyond the printed keys. Does that stabilize performance? How about when it is plugged in to a power supply vs a battery? How about when you’ve got earbuds or something else plugged into the line out? All these things affect the grounding of the PCB and stability of the keyboard.
If the random-note behavior is unaffected by any of these factors, then there’s something goofy with the circuit. Each key has an associated resistor; if any of them are damaged or loose it could create this behavior.
Macchiato inventor Brach may have other suggestions when he is back from holiday.