While copying from 𝓞 to 𝓟, Oliver Cowdery omitted two small words: the definite article the before multitudes and the not before fallen. The not is obviously necessary since in the next verse the narrative refers to Jesus Christ’s later visit to those who survived the terrible destruction at the time of his death (but definitely not to those who were killed by the destruction):
The 1981 LDS edition restored the not, but the RLDS text has maintained the reading in 𝓟 (which lacks the not).
Another example where Oliver Cowdery accidentally dropped the not is found in the description of this event later in 3 Nephi; that is, in 3 Nephi 8:20 we have the actual occurrence of the event prophesied in 1 Nephi 12:5, and the later text uses the very same language when it refers to the people “which had not fallen”:
In this instance, Oliver caught his error but not immediately. Later, probably when he was proofing 𝓟 against 𝓞, Oliver supralinearly inserted the not with somewhat heavier ink flow and a duller quill (a sure sign that the correction was not immediate). The 1830 edition (a firsthand copy of 𝓞 here in 3 Nephi) has the not, so we can be confident that 𝓞 (which is not extant here) read “which had not fallen”.
There are two other cases where not was accidentally omitted during manuscript transmission (see the discussion for 3 Nephi 20:45 and Ether 3:9). In both of these cases, it appears that Oliver Cowdery is responsible for dropping the not.
The definite article the should also be restored in 1 Nephi 12:5 (“I saw the multitudes which had not fallen”) since Nephi saw all those who survived the destruction. Of course, the the is found in the original manuscript and actually prepares the reader for the subsequent verse that uses only the pronoun them to refer to the survivors (“and he came down and he shewed himself unto them”).
Summary: In the RLDS text, the not before fallen needs to be restored since otherwise the following text makes no sense; and in both the LDS and RLDS texts, the definite article the before multitudes should be restored; both the and not are found in the original manuscript.