I noticed that Brazier was very passive, even deferential, to Kszchot when he passed just after 600m. It appears that once he knew he would be DQ'd anyway, he just caved in mentally. He was just going through the motions. A true competitor would have been mad at himself for the mistake, and probably run a PR on pure self-hate.
I'm not sure we will see the "killer instinct" with this kid, even as he matures. When you cave in at a major performance like this, it leaves damage. A great coach can help an athlete through this, but I am not sure that Brazier has that situation.