Given her only 800m run this year, and getting through the rounds of the 1500m to run a 4:03, it's hard to believe a healthy Mu couldn't get in very good shape between US Championships and Budapest.
That would suggest-
Either a health/injury problem - she did say she wasn't right for the 2022 World Championships, although she didn't specify what was wrong.
That she just doesn't want to compete, very probably because the pressure of an unbeaten record and expectations has taken a toll.
It somewhat reminds me of Herb Elliott who pretty much quite after the Rome Olympics season (he did a post-Olympics tour, and then did some lower key events or Cambridge when at college in England, generally avoiding the mile). He too had an undefeated record (at 1500m/mile) and in part not losing that record played a major part in Elliott's decision to retire at age 22. Rather like Mu, he was also someone for whom running was not his life.