Part of the issue imo is that there are at least two distinct definitions of 'YA" people use.
The first is content based; that YA is defined by a specific sort of prose style, plot, character approach, or focus on emotional drama and angst. And people will label any book with these qualities YA regardless of how the book itself is marketed or what its intended audience is.
The second is marketing; it’s YA because the publisher marked it as such.
And as much as YA is a marketing category and term, not all readers regard it as such and instead draw comparisons and similarities based on the content and style of the books rather than their market packaging.
For another reason entirely, Eric Nylund basically wrote the book on Halo’s universe outside the games. And his Halo Books were good.
Then they dumped him and hired Karen Traviss to shit all over his good work with her military fetish complex.