The Song of Roland was written in the 11th century, very loosely based on the Battle of Roncevaux Pass, like how the film 300 is based on the Battle of Thermopylae. The biggest difference between the actual battle and the Song of Roland is the enemy - the historical battle was fought against the Basque who took revenge on Charlemagne's weakened and retreating army after he destroyed much of Pamplona, while the enemy in the Song of Roland is a Muslim army. In both history and the song, Charlemagne's army is able to escape the ambush because the commander of the rearguard, Roland, decides to stand his ground to buy time for the rest of the army to escape. Although the Muslims in the Song of Roland are the enemy, they are not the real antagonist; that role is filled by Ganelon, a Christian knight and rival of Roland who informs the Muslim leaders of how they can ambush Charlemagne's army through a mountain pass because he is corrupted by his pride, greed, and envy of Roland.

Here is how the Song of Roland ends:

When the Emperor has dispensed his justice, And his great wrath has been appeased, He has Bramimonde christened. The daylight fades away, night has fallen, The King has gone to bed in his vaulted room. Saint Gabriel came from God to tell him: “Charles, summon the armies of your Empire! You shall invade the land of Bire,

You shall aid King Vivien at Imphe, The city the pagans have besieged, The Christians implore and cry out for you.” The Emperor would rather not go there: “God!” said the King, “my life is so full of suffering!” His eyes are brimming with tears, he tugs his white beard. Here ends the story that Turoldus tells.

As to why it ends so abruptly we can only speculate, but there are a few possible reasons.

The Song of Roland is what's called a chansons de geste, or 'song of deeds', and their format places little emphasis on the ending. These were typically long poems and songs following the exploits of a particularly noble hero, and many of them are essentially superhero stories. Because these songs were so long (some run to tens of thousands of lines, the Song of Roland is a mere 4200), they tend to be split into chunks that can be performed in isolation or as part of a series of performances. Manuscripts of chansons often have little notes in the margins along the lines of "break here", "ask for money here", "stop and resume the next day" etc. This means they are often pretty uninterested in wrapping everything up at the end. Instead, they resolve plot threads one at a time, usually one at the end of each act, and acts are often written to be able to stand on their own. Just as many modern TV shows follow a 'story of the week' format where each episode can stand on its own while tying into an overall plot over the entire season, many longer chansons de geste are told in the format of acts that can be performed in an afternoon that are, to an extent, able to stand on their own without having to listen to the whole song. A noteworthy example of this is the History of William Marshal, written in the early 13th century about the recently deceased (and much-loved) Earl William of Pembroke. It's tens of thousands of lines long, and while someone could listen to each performance over several days and get the whole story, they could just as easily only show up to the act where William Marshal defeats the French at the Battle of Lincoln, or the one where he tours the tournament circuits, or the one where he's stuck with King John while William's wife and John's lieutenant fight a war in Ireland, and each of those would be satisfactory experiences on their own. The Song of Roland is actually quite unusual in the interconnectedness of its different plots and acts; it is generally recognised to have an 'A' plot (Roland and Charlemagne's fight the Muslim army) and a 'B' plot (Ganelon's plot against Roland and its fallout) that interweave, when most chansons de geste move from one plot beat to the next in a more linear fashion, but even so it is split into four acts that are in turn split into further segments that may have had a brief intermission between them.

To quote the scholarly edition of the Song of Roland's introduction by Gerard J. Brault:

The four divisions are usually given as (1) the betrayal of Ganelon; (2) the death of Roland; (3) the punishment of the Saracens; and (4) the punishment of Ganelon. Rychner's detailed outline shows four subdivisions in the first part... eight in the second... four in he third... and three in the last part.

Because of this format, the ending of the whole tale is not actually that important; in many chansons de geste there was no reasonable expectation that a reader would even have the chance to listen to every single performance that made up a complete telling of the story. This meant that bringing everything together in a climactic conclusion was unwise, because many in the audience who missed a performance or two would be going "wait, when did that happen?" or "I don't' know what's going on". A lot of modern stories are based on techniques like Chekov's gun to establish setup and payoff, and that's good in stories where we can consume the whole thing in one sitting of a film or rewatch each episode on TV or streaming service of choice, but really backfires in a chansons de geste because the audience may not have had the opportunity to watch each act and see each setup for a payoff. So having a big, climactic conclusion is a really bad idea in a chansons de geste that already has multiple plot threads going at once.

As a result of this, the ending has already happened before the poem actually gets to its final few verses. Plot threads can't be left to hang around, so they get resolved efficiently. Before these two final verses, every plot in the tale is wrapped up. Roland and his companions died heroically half way through the song, and the rest of it follows and resolves the fallout. Roland's body is discovered and buried. Roland's death is avenged when Charlemagne duels and kills the Muslim ruler Baligant. Ganelon is exposed as a traitor and punished. The Muslim noblewoman Bramimonde converts to Christianity and is baptised. Supporters of Roland and Ganelon duel to the death, and Roland's friend Thierry is victorious over Ganelon's friend Pinabel. The story is already done. The A plot was handled when Charlemagne avenged Roland, and the B plot was wrapped up when Ganelon and his supporters are punished after Ganelon's trial.

If the story is already done, then there is nothing left to do except keep the audience wanting more. What you're looking at in the last two verses isn't really the end of the song, it's more like the post-credits scene. Although Turoldus' planned sequel where Charlemagne conquers the fictional land of Bire was never written, the next poem in the cycle was written in the early 12th century and follows Charlemagne's (entirely fictional) visit to the Holy Land and was clearly an attempt to deliver on the ending of the Song of Roland. The ending looks weird because, functionally, it's a post-credits scene rather than the actual ending of the story.

You might have some success at r/askliterarystudies as well.