In most versions of Yehoshua, when detailing the cities each Shevet gave to the Leviyim, Reuven's cities are missing. (Some versions add them after 21:35, but the conclusion of the Radak followed by the Minchas Shai is that they're not supposed to be there). Rav Hai Gaon, quoted by the Radak, addresses the question, but just says
אף על פי דהכא לא חשיב להו בדברי הימים חשיב להו
Even though it doesn't list them here, it lists them in Divrei Hayamim (1 6:63-64)
Does anyone explain why they're not listed in Yehoshua?