In my estimation, this is one of the most acute inconsistencies of amillennial theology in the book of Revelation.
“[The amillennial] approach does not fit the literary movement of Revelation. John pictures the period between Christ’s exaltation and return as the time of Satan’s banishment from heaven to earth, where he deceives the nations and persecutes the saints (Rev 12:1–17). By way of contrast, in 20:1–3 Satan is confined in the abyss, which means that he cannot deceive the nations “anymore” (eti), just as defeat in heaven meant that he had no place there “any longer” (12:8) and Babylon’s fall mean that life was not found there “anymore” (18:21–23). Satan does not deceive anyone during the millennium (20:4–6), but deception resumes afterwards (20:7–8; Mounce; Osborne). If the vision of Satan persecuting the faithful in 12:1–17 shows the present character of earthly life, the vision of Satan’s binding assures people that the present situation is not the final one. Evil will be defeated in ways that are not now evident (Boring; Giesen; Murphy)” [Craig R. Koester, Revelation: A New Translation with Introduction and Commentary, 785.]