My personal theory:
AMON or AMOS?
I think Amos was the original spelling in Matthew. This was neither Matthew's nor a scribe's error, but was related to the fact that Matthew's Gospel was written in Greek.
In Greek language many nouns end in -s, and this phoneme turns into -n when put into the accusative case. For example:
We can see something similar in Jesus' genealogy in Matthew. Most of the time, personal names end in -s in nominative case whereas in -n in accusative case:
Thus, it would be very easy for the name Amon to be transformed into Amos through assimilation. Amon would be considered the accusative form of Amos like most of the other names. We should keep in mind that the genealogy presented by Matthew gives the name in accusative form first and then in nominative form.
If this is the case, we cannot talk of a mistake or confusion, but can we still accuse Matthew of "Hellenizing" a Hebrew name? NO. This is because Matthew was not the only person to write AmoS although the original name of the Hebrew King was AmoN. I did research in the Greek version of the Hebrew Bible and came across something very surprising. Although the translators generally kept faithful to the Hebrew form of the name Amon, TWO verses in the Septuagint have the name AMOS rather than Amon:
[accordingly] as the word of God came to him in the days of Josias son of Amos king of Juda, in the thirteenth year of his reign.
ὡς ἐγενήθη λόγος τοῦ Θεοῦ πρὸς αὐτὸν ἐν ταῖς ἡμέραις ᾿Ιωσία υἱοῦ ᾿Αμὼς βασιλέως ᾿Ιούδα, ἔτους τρισκαιδεκάτου ἐν τῇ βασιλείᾳ αὐτοῦ· (Jeremiah 1:2)
In the thirteenth year of Josias, son of Amos, king of Juda, even until this day for three and twenty years, I have both spoken to you, rising early and speaking.
ἐν τῷ τρισκαιδεκάτῳ ἔτει ᾿Ιωσία υἱοῦ ᾿Αμὼς βασιλέως ᾿Ιούδα καὶ ἕως τῆς ἡμέρας ταύτης εἴκοσι καὶ τρία ἔτη καὶ ἐλάλησα πρὸς ὑμᾶς ὀρθρίζων καὶ λέγων (Jeremiah 25:3)
We see that only in the Greek version of Jeremiah the name AMOS is preferred over AMON. In the Greek version of another prophetic book the name AMON is maintained:
λόγος κυρίου ὃς ἐγενήθη πρὸς Σοφονιαν τὸν τοῦ Χουσι υἱὸν Γοδολιου τοῦ Αμαριου τοῦ Εζεκιου ἐν ἡμέραις Ιωσιου υἱοῦ Αμων βασιλέως Ιουδα (Zephaniah 1:1)
We do not know exactly why Jeremiah LXX contains AMOS rather than AMON. A possibility is that in the Masoretic version of Jeremiah the name AMON refers to a false god rather than a king:
The Lord God of Israel who rules over all says, “I will punish Amon, the god of Thebes. I will punish Egypt, its gods, and its kings. I will punish Pharaoh and all who trust in him.
Yet the interesting thing is that this verse is missing from the Septuagint!
Conclusion: Matthew quoted the name of the Hebrew king (Amon) from Jeremiah LXX (AMOS). Although we cannot know exactly why he did so, we know that the occurrence of the name Amon as AMOS in Matthew is by no means a mistake.