Both made and manifest begin the same (with the two letters ma), which may have led the 1837 typesetter to accidentally skip the made. The deletion of made was not marked by Joseph Smith in the printer’s manuscript. Elsewhere the Book of Mormon text has 14 occurrences of the passive “to be made manifest”, three of “to be manifested”, but none of “to be manifest”. Nor did Joseph Smith ever attempt to change any of the 14 other occurrences of “to be made manifest” by deleting the made. Thus we have an example here in 1 Nephi 22:2 of an accidental error creating the only occurrence of “to be manifest” in the text. In accord with the printer’s manuscript, the 1908 RLDS edition restored the original reading, “they were made manifest”.
A similar error where made was accidentally skipped occurred in Alma 37:21 of the original manuscript. There Oliver Cowdery initially wrote “all their wickedness and abominations may be manifest”. Oliver immediately caught his error, erased manifest and overwrote the erasure with made, and then wrote manifest immediately afterwards on the same line (see line 9 on page 297 of 𝓞).
Summary: Follow the original reading of “to be made manifest” in 1 Nephi 22:2; the 1837 reading of “to be manifest” is a unique reading and otherwise nonexistent in the text.