The earliest text here reads “for thus did thou manifest thyself unto thy disciples”. In the 1879 LDS edition, the form did was replaced with didst since the subject for the verb is thou, which requires “thou didst” in standard Early Modern English. (The RLDS text has, however, retained the earlier reading with did.) As explained under 2 Nephi 24:12, there are cases of thou in the original text where the associated past-tense verb form lacks the expected -(e)st ending. In such cases, the earlier reading without the ending will be maintained or restored (as the case may be); here in Ether 12:31, the critical text will restore the original did.
Summary: Restore in Ether 12:31 the original did in “for thus did thou manifest thyself ”; although didst is standard here, usage elsewhere in the original text supports the occasional second person singular past-tense verb form without the -(e)st ending.