Here's the only thing when people say BYU Admissions is a crapshoot...
They already posted the statistics for admitted freshmen for 2021-22. Stats:
Middle 50% GPA: 3.86-4.00
Middle 50% ACT: 28-32
So it's clearly not the norm that a student with a 3.5 GPA gets in while a 4.0 does not. The GPA and ACT are very high. I think there are exceptions due to the other parts of the application (recommendations, essays, etc.) but that's not the norm, otherwise the GPA and ACT scores would be all over the place.