The issue for me is that Mangum played well in 2015 with a PER similar to Hall 2007. As far as I can tell, his PER this year is worse than any BYU QB that's started more than a few games.
It isn't so much that he's a disappointment, it's that he's gotten much, much worse. By contrast, Heaps PER his sophomore year was just a couple of points of his PER as a freshman. Again, nothing like we're seeing here.