ציטירנדיקער פּירוש אויף ירמיהו 49:6

Rabban Gamaliel said to him: the verse says, "But afterward I will bring back the captivity of the children of Ammon," (Jeremiah 49:6) they have already returned. Rabban Gamaliel actually accepts Rabbi Joshua's argument but responds that there is another verse that shows that God brought back the Ammonites from their exile. According to Rabban Gamaliel the verse from Jeremiah does not refer to the "end of days" as do the verses regarding the Moabites (Jeremiah 48:47) and Elam (49:39). Rather, God has already restored the Ammonites, so the prohibition from Deuteronomy applies again.

and your cities will not return [Heb. תָשֹּׁבְנָה,] not as I said about Egypt (above 29:14): “And I will return the captives of Egypt”; (Jer. 49:6): “And afterward, I will return the captivity of the children of Ammon.” And this is what Malachi (1:3) said, “And I hated Esau, and I made his mountains desolate,” for I made them destined for everlasting desolations.