Here the 1874 RLDS edition omitted the repeated a before the conjoined adjective true. And later, the 1920 LDS edition independently omitted the repeated a (there the omission was definitely intended since it was marked in the 1920 committee copy). As noted in the previous discussion, for that edition the original more short was changed to shorter. So what we have here in the 1920 edition is a change from “a more short but a true account” to “a shorter but true account”. David Calabro points out (personal communication) that a change to “a more short but true account” (that is, the removal of the repeated a while maintaining the original more short ) would have led to a possible misreading regarding the scope of more, as if the phrase meant ‘a shorter but truer account’.
Elsewhere in the text, there are similar conjunctive constructions for which the repeated a has been removed from the text, as in these cases where the first conjunct is a periphrastic comparative with more:
In general, the loss of the repeated indefinite article a has occurred fairly frequently in the transmission of the Book of Mormon text (for a list of examples, see under conjunctive repetition in volume 3). For each case, as here in 3 Nephi 5:9, the critical text will restore the original repeated a.
Summary: Restore the original repeated a in 3 Nephi 5:9, “a more short but a true account”.