And I appreciate the brain storming!
The other thing I'm going to try is to move the sensor from the passenger's side tube to the driver's side tube. I can rule out anything strange about one side since the 2 are isolated.
The only other thing I've contemplated is whether there is anything about using oval tubing that could cause the sensor to read strangely. I have googled to try and see if anyone else has reported anything like this, and so far I haven't found anything. I get the impression that having the sensor out of the middle / direct path of the flow can be good because there is less likelihood of any liquid hitting the sensor.