Samuel the Lamanite had described many signs which would attend the birth and the death of Christ. Among those that would come to pass at the time of his death was that many of the faithful who died bad from the days of Adam to the meridian of time would rise from the grave at the time that Jesus did (see Helaman 14:25-26).
That is, Samuel spoke of the initiation of the first resurrection. These things did indeed come to pass. Of the events in the Old World, Matthew recorded:
“And the graves were opened; and many bodies of the saints which slept arose, and came out of the graves after his resurrection, and went into the holy city, and appeared unto many” (Matthew 27:52-53).
The same miraculous events took place in the New World but, for some reason, the Nephites had failed to record them. Under direction front the Savior, Nephi now added this important information to the records. Presumably this was added to the large plates.
Either Nephite recorded only the general statement or Mormon, the prophet-abridger of the large plates, chose not to include in his record the details of the resurrection of the American Saints.