Here we have a virtually immediate correction in 𝓟, where scribe 2 supralinearly inserted the subordinate conjunction that (there is no change in the level of ink flow). Since either reading is possible in English, there is no strong motivation for scribe 2 to have edited the text here. Generally, the text has examples with and without that, as in the following parallel examples found in the same chapter of Alma:
In all such cases where the that is optional in English, the critical text will follow the earliest textual sources. For further discussion as well as examples involving a variety of verbs, see under that in volume 3.
Summary: Maintain the subordinate conjunction that which was virtually immediately inserted in Mosiah 27:18 by scribe 2 of 𝓟.