The original manuscript definitely has the singular tradition. In the printer’s manuscript, Oliver Cowdery initially copied this word as a plural, but then crossed out the plural s. The crossout appears to be virtually immediate since there is no change in the level of ink flow. Moreover, this crossout on line 23 of page 250 of 𝓟 is like other crossouts on this page of 𝓟, including the crossout of of seven lines earlier on the page where Oliver supralinearly inserted an ampersand to replace the of (the ampersand is definitely Oliver’s, so we can be confident that the crossout is also his).
Of course, the use of the singular was here in Alma 31:16 is not conclusively supportive of the singular tradition since we do have cases where the plural traditions takes a singular verb form in the earliest text. For discussion of this possibility, see under Alma 9:16. For further discussion regarding the grammatical number for tradition(s), see under Mosiah 1:5.
Summary: Accept the singular reading tradition in Alma 31:16, which is the reading in 𝓞 as well as the corrected reading in 𝓟.