At the start of a Full GC we call ConcurrentMark::abort() to notify the concurrent marking operation that might be in progress to stop what it's doing. CM::abort() clears the next marking bitmap:

// abandon current marking iteration due to a Full GC
void ConcurrentMark::abort() {
  // Clear all marks to force marking thread to do nothing
  _nextMarkBitMap->clearAll();
  ...
}

This is not really necessary, at least not because of the explanation in the method. The CM workers should have yielded before the Full GC safepoint could start, therefore they will not observe that the bitmap was cleared.

Having said that, not doing the clearing does introduce infrequent failures during the first GC pause after a Full GC that aborts a CM cycle. So, we should first get to the bottom of why this is. But, it will be good to avoid the bitmap clearing at the start of the Full GC and let the CM thread do it concurrently.