While the MAX31865 can support either 100ohm or 1kohm RTDs, the Adafruit breakout comes in two versions with different reference resistors specific to each type of RTD.
If you are unsure which one you have, you can check the value of the reference resistor by reading the label. See below for details.
Also, be sure to change the value for RREF in your code to match.
If marking is 4300 or 431 its PT100
If marking is 4301 or 432 its PT1000