Here Oliver Cowdery initially wrote forth in 𝓟 but immediately crossed it out and wrote thither inline. Thus 𝓞, not extant here, must have read “went thither” rather than “went forth”. As might be expected, there are many examples of “went forth” in the original text (85 of them). But here in Mosiah 18:6, we have the only occurrence in the entire text of “went thither”. There was clearly no reason for Oliver to have edited the highly expected “went forth” to a unique reading, “went thither”.
Summary: Maintain in Mosiah 18:6 the unique reading “went thither”, the corrected reading in 𝓟.