Here the 1953 RLDS edition changed the singular affliction to the plural afflictions. This change was unnecessary and may have been accidental since nowhere else did the 1953 edition make any change in the number for affliction. Elsewhere the Book of Mormon text has only the plural afflictions when premodified by great:
Even so, the singular great affliction is possible, as in the following King James passage:
More generally, affliction takes both singular and plural forms throughout the Book of Mormon. For discussion regarding the phrase much affliction(s), see under 1 Nephi 16:35. The evidence there argues that in each case we should rely on the earliest textual evidence in determining the number for affliction. Thus the critical text will maintain the singular great affliction here in Mosiah 11:27, even though it is possible that a plural s could have been lost in the early transmission of the text.
Summary: Retain the singular affliction in Mosiah 11:27 (“who is the Lord that shall bring upon my people such great affliction”).