The original manuscript has burnt, an alternative past participle form for burned. When Oliver Cowdery copied the text from 𝓞 into 𝓟, he replaced burnt with the more common burned. Since either verb form will work, we follow the earliest textual source (the original manuscript) and restore burnt.
Elsewhere in the text, there are five occurrences of burnt offerings, which is of course what we expect when the past participle is used as an adjective. Otherwise, when the text uses the verb burn either in the simple past or as a verbal past participle, we have burned. Only here in 1 Nephi 12:4 do we get burnt as a verbal past participle. It is quite possible that this form represents a dialectal intrusion on the part of either Joseph Smith or scribe 3 of 𝓞.
Summary: In 1 Nephi 12:4, maintain the past participle burnt (the reading of 𝓞), even though it is quite possible that burnt is an error for burned.