Oliver Cowdery initially omitted the word behold here in the original manuscript; somewhat later he supralinearly inserted it (the ink flow is slightly heavier), perhaps when he read the text back to Joseph Smith. Since either reading will work, there would have been no motivation for Oliver to consciously emend the text here. There are ten other examples of this kind of construction in the original text (“it came to pass that <<em>when-clause> behold ”), as in 1 Nephi 7:18: “and it came to pass that when I had said these words / behold the bands were loosed from off my hands and feet”. Here in Alma 46:21, the critical text will maintain the corrected reading in 𝓞.
Summary: Maintain in Alma 46:21 the corrected reading in 𝓞, “behold the people came running together”.