Here Joseph Smith changed the singular is to the plural are in his editing for the 1837 edition. Another possibility would have been to change the plural judgments to the singular (“according to his judgment which is just”). It is even possible that the original manuscript read judgment and that Oliver Cowdery accidentally added the plural s while copying (𝓞 is not extant here). Elsewhere in the Book of Mormon text, we have more occurrences of his judgments (six times) than his judgment (two times)—and either the singular judgment or the plural judgments can occur with the predicate adjective just:
Here in Omni 1:22 the predicate adjective just occurs in a relative clause. In this part of the small plates of Nephi, there are two similar instances of subject-verb disagreement. In both cases, there is a plural antecedent, the relative pronoun is which, and the verb is the singular is:
Thus the use of the singular is in a relative clause can occur with an immediately preceding plural antecedent (such as judgments in Omni 1:22 and all things in Omni 1:25 and the Words of Mormon 1:7). The critical text will restore such instances of subject-verb disagreement. For further discussion, see under 1 Nephi 4:4 as well as under subject-verb agreement in volume 3.
Summary: Restore the singular is in Omni 1:22, Omni 2:25, and the Words of Mormon 1:7; these examples show that in a relative clause this singular verb form can be associated with a plural antecedent.