Book of Mormon Critical Text discusses the longer ending of Mark and why scholars tend to believe it is not original to the Gospel of Mark.
Book of Mormon Critical Text: A Tool for Scholarly Reference, 3 vols. (Provo, UT: Foundation for Ancient Research and Mormon Studies, 1987), 3:1182n263
KJ Mk 16:18 || Acts of Pilate 14:1; Mk 16:9-20, the Longer Ending of Mark, is attested in unicalACDKXiDeltaThetaPi (lengthened vs 14 in unicalW; with other varr in unicalLPsi 099 0112) minuscule manuscripts 28 33 and Family 13 (in 22 and Family 1 with doubts shown) SyrChp Syp, Sa’idic Boharic and Fayyumic Coptic, Diatessaron, Apostolic Constitutions, Irenaeus Tertullian, etc., but is absent from unical%B minuscule manuscript 304 Old Latink SyrS, Clement of Alexandria Origen Eusebius Jerome Ammonius, etc.; the consensus is that the Longer Ending was not originally part of Mark, but that it is part of an unknown and authentic ancient gospel, and thus is included in UBS3 in double brackets (Metzger, Textual Commentary, 122-126).