SEN0313 Ultrasonic Sensor sending inaccurate data after a few days of outdoor use

I have been using two SEN0313 (A01NYUB) ultrasonic distance sensors to measure the waterlevel of a small river. The sensors were mounted on a bridge, facing straight down onto the water surface at 90° angle. They take one measurement every 15 minutes.
After measuring correctly for about 1.5 days, one of the sensor suddenly started sending wrong values.
(Expected distance ~1.8 m, the sensor started giving values of ~0.8 m).
The strange thing is that it's not random data, but very consistenly similar values around 0.8 m.

You can see the measurement plotted over time below

The sensor has clear, unobstructed line of sight to the water surface. There is most definitely no object at 0.8 m. There is only the water surface which was never at that height during the measurement period. (Around 1.8 m as in the beginning is plausible)

After noticing the strange values, I unmounted the sensor and tested it indoors and got similar results, leading me to believe the sensor is somehow broken. (Although nothing is visually broken or damaged)
I performed the same indoor test with a third, brand new SEN0313 and it performed flawlessly for several days.

So my question is:
Why and how did the sensor break outdoors? According to the datasheet, it's rated IP67 and up to -15°C. It definitely was not colder than -15°C during the period and the sensor only experienced light rain. Wheres problem?