Here in the original manuscript, Oliver Cowdery initially omitted the second instance of the subordinate conjunction that. Virtually immediately he supplied it (there is no difference in ink flow for the supralinearly inserted that). The occurrence of that is very common after “I say unto you”; in fact, there is an example earlier in this verse: “yea I say unto you that because they shall sin against so great light and knowledge”. But the that is not required in the text; we have, for instance, examples like Alma 41:10: “behold I say unto you : wickedness never was happiness”. For each case of “I say unto you”, the critical text will rely on the earliest textual sources in determining whether the that is there. Here in Alma 45:12, the critical text will follow the corrected reading in 𝓞 (“yea I say unto you that from that day …”).
Summary: Maintain in Alma 45:12 both instances of that following “yea I say unto you”, the invariant reading in 𝓞 for the first instance and the virtually immediately corrected reading in 𝓞 for the second instance.