In this passage the 1840 typesetter inserted an extra when before “it came to pass”, undoubtedly in anticipation of the following when in “when Ammon had made ready the horses and the chariots”. Surprisingly, this obvious typo persisted in the RLDS textual tradition until it was removed in the third RLDS edition (1908). There are no other examples in the text of “come to pass” preceded by the subordinate conjunction when. There is one example of the conjunction before preceding “come to pass”, namely in 1 Nephi 20:5 (a quotation of Isaiah 48:5): “before it came to pass / I shewed them thee”. As expected, the before is not repeated here.
Summary: Maintain the earliest reading in Alma 18:12 without the intrusive when that was inserted in the 1840 edition before “it came to pass”.