First, is when we become the number 1 seed, the first game he played he lost.
And now this time, but I have to check though, maybe he is injured or something or just lost (again) his mental focus. if this will be frequent though then most likely he will not stop as number 1 for a long time like the big 3 before him.
It does not appear that he has any problems or injuries. But he should be thoroughly checked. But in my opinion, it was just a lapse in concentration. Maybe he cannot cope with the pressure of being at the top and maintaining his position. I think we should cut him some slack because he is just a young kid. He will learn how to handle pressure and how to approach future matches when he is older.
There are also a lot of pressure in his shoulder, but he seems to beat the odds and achieved his goal. So he is mentally focus because if he is not then he will be the likes of other who is making a good run until they stop improving like Tsitsipas. So he can't be this kind of reasoning, overconfident? nah, he can't do that, lost focus? I doubt that. Again the pressure is already in his shoulders way before, as he is being compared ot Nadal like 6-8 months ago, so he can handle pressures like coming back from 2 sets down in a 5 setter and win the game. This is just very uncharacteristic of him, just saying.
In this case, I have to agree with you. Because he is only a kid, he might feel a lot of pressure to prove he is actually worthy of being number one. The pressure can often lead a man to make mistakes that he normally wouldn't have made. Some people are very quick to compare a young player with some of the legends of this generation when it comes to evaluating young players.
It is true that he is one of the leading players in the world right now and the number one grant player in the world right now. But that does not mean he is already at the level of Rafael Nadal or Rodger Federer. At the same time, it is also true that being the best in the world does come with some consequences, and being compared to the best is just one of them.
His only fault is that he is going too much for his strokes, he needs to learn how to finish points without hitting the lines sometimes. He believes in himself maybe a bit too much, and needs to add a bit of safety margin to his points. He will learn that will time and then he will lower unforced errors count and become even tougher to beat.
Alcaraz has his own style and sticks to that style and if he wants to be on top of the world every year he will need to implement more playing styles and not just hitting the corners. Look at Djokovic has a lot of different styles and i think if they allow Djokovic to play in all tournaments he will retain the first place fast and make even more records.