You may be right. I'm still at 97%-98% on all sensors 10am and may not drop out of 90's for another day or so, pushing that 60 hours of saturation before rain stops.
That being the case, then it's all about whether the 75 reads significantly lower in dew point once humidity drops below 80% for awhile. I've seen it take anywhere between 6 to 48 hours for a good sensor to automatically recalibrate after being in high humidity for more than 24 hours (Sensirion may say 60 hours, but it's more like 1-2 days). If the air continues flirting with the 80 percentiles then it can take a long time (days) for the recalibration to occur. If humidity falls into the 60s or lower, then it usually takes just a matter of hours to sort the good from the bad sensors.
My "good" 31 actually reads worse than my bad 31 before it recalibrates. I'm talking an average of 3.5 to 4F (10+%) high in dp when the humidity first starts dropping below 80%. In case anyone doubts this, I've run tests with an accurate device (factory tested to +/-2% and confirmed by testing it at my local airport).
The good news is that both your sensors are still getting up to 97-98%. That tells me they are relatively healthy. Though dendrite would have said the same about his 75 until it suddenly went bad.
I should point out that max humidity isn't always an indication of whether a sensor is accurate throughout the entire range. My "good" 31 will now only max out at 95% when the air's been saturated for hours. 94% is more common. Generally, though, it's within 2% when humidity is consistently below 80%.