Personal opinion here.
Noah strikes me as a great kid - but the operative word here being "kid". Outside of sport he still has a lot of, how should I say this "youthful" pastimes such as his video games, painting, playing with lego and his Dragonball Z/anime interests.
Now that this is definitive, but when I put this together with his social media/twitter outputs I get the impression of a child very much in a grown mans body. I'm trying to say this in way that isn't disparaging but it is what it is. And I believe that to a large degree this "youthful" approach to life was what made him so successful and bulletproof when he first entered the pro ranks.
But just as a very youthful mindset can work so well when things are going well, it can do the opposite when you face adversity. We all remember being kids and what it's like when things go well right? Well also remember the first time things don't and how difficult it can be to comprehend that. The understanding that life has ups and downs and how to deal with it is all part of maturity.
I don't think anything really happened to Noah per se - but I think things got real with the pressure and expectation of winning the Olympics and he is not mentally equipped to handle it. I think this contributed to the anxiety/depression and whatever the ramifications (some people mentioned the weight gain etc) of that was. In 2019 I saw a happy, care-free kid out there dancing and doing his thing - in 2021 I saw someone trying to the do same stuff but in a more forced and contrived way. He just didn't look happy and looked more troubled and skittish than anything.