Waterproof vs External Humidity sensors in Iphone X

After 6 months of use, my Iphone XS Max had some issues with the touch screen in the upper left side. I visited the apple service store and the technician diagnosed the device had some software issues. When they open the phone for the repair they "detected" that one of the external humidity sensor located outside of the phone and close to the charge port was lightly colored to yellow, meaning that the phone had some moisture and because of that any quality or software were be not covered by the warranty of applecare.

1.- That was the second claim to applecare regarding this phone. The first one was because i cracked the backside of the phone and apple gave me a "new device" that clearly could be a refurbished phone explaining the issues with the touch system.

2.- Clearly, the phone had a software issue that was diagnosed by the test the technician ran during the service.

3.- I never EVER exposed the device to the water, it was verified because the phone only had the EXTERNAL sensor lightly activated but not any other of the external neither an internal damage.

The question is ... how apple can sale a phone that is waterproof and put an external sensor that can be activated with the weather conditions and then reject the warranty and refuse to fix the quality issues that they had during production?

These external sensors will be always activated especially if you live in cities with a high humidity levels however as the sensor are externals, it doesnt mean that the phone was exposed to a liquid or had any interior damage. Why this incident with software is counted as damaged when it is clearly a device issue?

I think is no very ethic placing a sensor in the outside of the phone to avoid taking responsabilities of the issues that must be covered by the warranty.