We have already seen one other example (in 1 Nephi 2:16) where Oliver Cowdery, in his copying from 𝓞 to 𝓟, accidentally added the auxiliary verb do. Here in 1 Nephi 18:11, Oliver seems to have been influenced by the use of the auxiliary verb do earlier in this verse: “Laman and Lemuel did take me and bind me with cords and they did treat me with much harshness”. Elsewhere the text has nine occurrences of the simple past-tense suffered and four occurrences of did suffer, so either reading is theoretically possible. In each case, we let the earliest textual sources determine the reading. In 1 Nephi 18:11, 𝓞 has suffered rather than did suffer.
Summary: Restore the reading of the original manuscript in 1 Nephi 18:11 (“the Lord suffered it”).