Thanks for replying. I've made some progress narrowing this down. Board is connected via usb and no external components.

Using this code:

void setup() {
  Serial.begin(115200);
  while (!Serial) {;}
}
void loop() {
  int sensorValue8 = analogRead(A8);
  int sensorValue9 = analogRead(A9);
  int sensorValue12 = analogRead(A12);
  int sensorValue13 = analogRead(A13);

  Serial.print(" A8: ");
  Serial.print(sensorValue8);
  Serial.print(" A9: ");
  Serial.print(sensorValue9);
  Serial.print(" A12: ");
  Serial.print(sensorValue12);
  Serial.print(" A13: ");
  Serial.println(sensorValue13);
  delay(1000);  
}

With nothing connected to pins I get:
A8: 77 A9: 59 A12: 78 A13: 71
A8: 75 A9: 59 A12: 78 A13: 71
A8: 75 A9: 59 A12: 77 A13: 70
A8: 74 A9: 57 A12: 77 A13: 69
A8: 73 A9: 55 A12: 77 A13: 69
A8: 71 A9: 55 A12: 77 A13: 63
A8: 71 A9: 54 A12: 75 A13: 63
A8: 71 A9: 55 A12: 75 A13: 69
A8: 73 A9: 55 A12: 76 A13: 63
A8: 75 A9: 59 A12: 77 A13: 71

With th 3.3v pin connected to A12 or A13, no change.

With th 3.3v pin connected to A8 I get this:
A8: 1023 A9: 199 A12: 1023 A13: 238
A8: 1023 A9: 201 A12: 1023 A13: 239
A8: 1023 A9: 201 A12: 1023 A13: 239
A8: 1023 A9: 199 A12: 1023 A13: 239
A8: 1023 A9: 197 A12: 1023 A13: 237
A8: 1023 A9: 191 A12: 1023 A13: 236
A8: 1023 A9: 189 A12: 1023 A13: 234
A8: 1023 A9: 183 A12: 1023 A13: 232

With 3.3v on A9 I get this:
A8: 73 A9: 1023 A12: 191 A13: 1023
A8: 70 A9: 1023 A12: 190 A13: 1023
A8: 69 A9: 1023 A12: 190 A13: 1023
A8: 73 A9: 1023 A12: 190 A13: 1023
A8: 77 A9: 1023 A12: 191 A13: 1023
A8: 85 A9: 1023 A12: 191 A13: 1023

IF I comment out the analogRead and prints for A8 and A9:

void setup() {
  Serial.begin(115200);
  while (!Serial) {;}
}
void loop() {
//  int sensorValue8 = analogRead(A8);
//  int sensorValue9 = analogRead(A9);
  int sensorValue12 = analogRead(A12);
  int sensorValue13 = analogRead(A13);

//  Serial.print(" A8: ");
//  Serial.print(sensorValue8);
//  Serial.print(" A9: ");
//  Serial.print(sensorValue9);

  Serial.print(" A12: ");
  Serial.print(sensorValue12);
  Serial.print(" A13: ");
  Serial.println(sensorValue13);
  delay(1000);     
}

and then apply 3.3v to A12 I get this:
A12: 1023 A13: 283
A12: 1023 A13: 251
A12: 1023 A13: 217
A12: 1023 A13: 200
A12: 1023 A13: 223
A12: 1023 A13: 255

3.3v on A13 give this:
A12: 195 A13: 1023
A12: 175 A13: 1023
A12: 171 A13: 1023
A12: 190 A13: 1023
A12: 209 A13: 1023

And 3.3v on A8 or A9 has no effect.

So it only seems to be a problem when reading A8/A9.

Uh oh!

GIGA R1 - analogRead(A12) returns A8 value #758

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