instability of the board source can cause failures when running the ADC calibration. One way to mitigate this is to run the calibration multiple times, e.g. as it is done in RTD demo examples. Also be sure reserved bits in CALBISTREG are not modified. Writing to this field can cause ADC to operate unreliably.
you can try to perform ADC selftest as well to check the ADC. If you have issue still share the simplified project you use. Specify the HW used as well.
