The MAX31656 contains a mains frequency notch filter, set to 60Hz by default. This should be changed for applications in 50Hz regions. The arduino library for this breakout board allows for selecting 50Hz:

https://github.com/adafruit/Adafruit_MAX31856/blob/master/Adafruit_MAX31856.cpp#L189

However this feature is missing in the Circuitpython library.