Oliver Cowdery initially wrote the text without again, then somewhat later supralinearly inserted the word again with heavier ink flow. There are two possibilities: either (1) Oliver edited the text, or (2) while checking the printer’s manuscript, he corrected it so that it would agree with the reading in the original manuscript. The text could read without again, which suggests that there is no strong motive for Oliver to have edited the text here. Therefore, the more plausible possibility is that Oliver was correcting the text to agree with the reading of the original manuscript (which is no longer extant here).
Later on in 2 Nephi, we have a second reference to how the plates would be resealed after Joseph Smith’s translation was complete:
Here again is once more used to refer to what Joseph Smith would do with the plates after finishing the translation: he would seal them up again. This nearby reading thus supports the corrected reading in 2 Nephi 27:22.
Summary: Retain in 2 Nephi 27:22 the reading with again (“then shalt thou seal up the book again”), Oliver Cowdery’s correction in 𝓟 (which apparently made 𝓟 agree with 𝓞).