This anomaly applies to IC Rev. Engineering A, build codes QIAA-AA0.
The LFRC oscillator non-calibrated frequency tolerance (ftol_uncal_lfulp) in ULP mode is not according to specification.
LFRC in ULP mode.
The LFRC ULP oscillator non-calibrated frequency is unreliable.
Do calibration after each power-up, after changing mode between normal mode and ULP mode, and if the temperature changes more than 40°C.