This is another NES inconsistency I've just discovered. If you kill a segment of a Moldorm (not the whole Moldorm, just one of its segments), leave the room immediately, then come back, they never come back. On the NES version, if you kill the segment of a Moldorm and leave and re-enter the room, it comes back. They only die for good if you kill one and leave/re-entering the room on the NES.
Bug with segmented enemies
Fixed.