A well-designed medium temperature refrigeration system will maintain both a proper case temperature and relative humidity (rh). The preservation of food depends not only on the temperature, but also on the relative humidity within a case.
Actually, frost on the suction line only indicates that at the location where the frost is present, the suction line piping is below the dew point temperature of the surrounding ambient air and at or below 32°F. That’s it. It is neither an indication of a properly operating system nor a system defect.