From Graybeard:
In one instance, it was noted the altitude indicated a drop of 300 feet, meaning the ADR was still outputting valid altitude calculation, but perhaps without airspeed trim.
Right, an uncorrected altitude has an error magnitude of a few hundred feet. That may be considered unacceptable for TCAS since RAs occur for an anticipated vertical separation of only 700 ft (above 20,000 ft).