Here the original text reads perfectly normally as “when he saith that he repenteth”. The 1840 compositor accidentally moved the subordinate conjunction that before the word saith, giving the quite impossible “when he that saith he repenteth”. Even so, this reading was copied into the 1858 Wright edition. Editors for the 1874 RLDS edition (which derives from the 1858 Wright edition but with frequent reference to the 1840 edition) tried to emend this strange reading by moving the that before the subject pronoun he. The resulting reading is even more difficult: “when that he saith he repenteth”. Finally, the 1908 RLDS edition restored the correct reading to the RLDS text since 𝓟 reads “when he saith that he repenteth”.
Summary: Maintain in Mosiah 26:31 the placement of the that after the verb saith (“when he saith that he repenteth”).