PH Sensor SEN0161 Opposite Measurements

Hello,

We have got a new sensor and while trying to calibrate, the sensor is detecting pH 7 on pH4 buffer solution and it is detecting pH4 on pH7 buffer solution. Also, all other measurement are the opposite, acids are above pH 7 and alcaline are under pH 7!

Please advise. I used the library and code from the wiki page: https://wiki.dfrobot.com/Gravity__Analog_pH_Sensor_Meter_Kit_V2_SKU_SEN0161-V2