In the original Book of Mormon text, the prepositional a is usually found as part of the main verb phrase, as in 1 Nephi 8:28: “those that were a scoffing at them”. The 1830 typesetter typically removed these instances of the prepositional a. There are a couple instances of the prepositional a after the past-tense or past-participial verb form found, here in Alma 62:10 and also once earlier in the text:
In that case, the 1830 typesetter did not remove the a; the removal of the a occurred in the subsequent (1837) edition. The critical text will restore these two instances of “find someone a doing something”. On the other hand, there are four instances in the earliest text of “find someone doing something”—that is, where the text is lacking the prepositional a:
So either construction, with or without the prepositional a, is possible in the original text. For further discussion, see under prepositional a in volume 3.
Summary: Restore the prepositional a in Alma 62:10 (“whosoever was found a denying their freedom”) as well as in Alma 21:11 (“he found Muloki a preaching the word”).