For the 1837 edition, Joseph Smith edited “this our afflictions” to the singular “this our affliction”. Given the singular is as well as the this, we expect the singular affliction. There is also evidence that Oliver Cowdery sometimes accidentally wrote the plural afflictions instead of the correct singular:
So it is quite possible that the earliest reading in Mosiah 7:23, “this our afflictions”, is an error for “this our affliction”.
Elsewhere the current LDS text uses singular nouns when immediately preceded by this and a possessive pronoun:
this my people Mosiah 1:10 (plus seven more occurrences)
this my joy Mormon 2:13
this thy people Ether 3:3
this our faith Alma 44:3
this our weakness Alma 56:19
this our desire Alma 56:23
this our march Alma 58:27
this your way of sin Helaman 9:21
this their great fear Alma 27:23
this their time of affliction Alma 55:31
this their great wickedness Helaman 4:13
this their humility Helaman 11:11
For most of these cases, the singular noun is expected even if the this were not there. In two cases, the singular occurs in the earliest textual sources (“this our weakness” in Alma 56:19 and “this our desire” in Alma 56:23). But in two of the above cases (marked in bold), the earliest text actually reads in the plural:
These two examples show that the plural is possible. The critical text will therefore restore the plural “this our afflictions” in Mosiah 7:23.
Summary: Restore the plural afflictions in Mosiah 7:23 (“this our afflictions”); two other instances of this type of plural construction can be found in the earliest text.