So the i2c test code stopped working abruptly after adding logic to set the digital pots output randomly every 2 seconds. Checked and double checked all the PCB connections and reworked most of the solder joints. Got the scope out and probed SCL and SDA. SDA had some small glitches but was staying at 3.3V. Tried another ESP32, same results.
Foiled again by the Arduino IDE! I had changed the ESP32 board to for another project but didn't change it back before resuming this testing. The other board must have had a different default pin for SDA. Everything worked fine after setting back to Node32s board. (I wonder if the 2.0 IDE will store board type in the project....)