Contradiction with Deuteronomy 2:36
This verse says that not a single city was too strong for the Israelites, which contradicts with the implication in Numbers 21:25 of cities being taken by force.
Deuteronomy 2:36: From Aroer, which [is] by the brink of the river of Arnon, and [from] the city that [is] by the river, even unto Gilead, there was not one city too strong for us: the LORD our God delivered all unto us:
Contradiction with Joshua 13:17
This verse lists cities as part of Reuben's inheritance, contradicting the notion in Numbers 21:25 that they were taken and settled solely by the Israelites.
Joshua 13:17: Heshbon, and all her cities that [are] in the plain; Dibon, and Bamothbaal, and Bethbaalmeon, [Bamothbaal...: or, the high places of Baal, and house of Baalmeon]
Contradiction with Judges 11:26
This verse questions why Israel took such a long time to claim the land if it was theirs, contradicting the idea in Numbers 21:25 of an immediate claim.
Judges 11:26: While Israel dwelt in Heshbon and her towns, and in Aroer and her towns, and in all the cities that [be] along by the coasts of Arnon, three hundred years? why therefore did ye not recover [them] within that time?
Contradiction with 1 Kings 4:19
This verse suggests another king ruling over the cities, contradicting Numbers 21:25's narrative of conquest by Israel.
1 Kings 4:19: Geber the son of Uri [was] in the country of Gilead, [in] the country of Sihon king of the Amorites, and of Og king of Bashan; and [he was] the only officer which [was] in the land.