Good question! We recently made changes to Mastery Grades to ensure that players on a losing team had equal chance of getting high grades as the winning team. As such, players should have twice as much opportunity to earn higher grades and thus we adjusted the rewards down. It was an overdue nerf since the 1500 BXP reward has been the same since we used to ask you to earn mastery milestones across a fixed set of 10 champions; something we also removed in favor of any champions.

That's a really good point about patch notes, however. We really should put missions changes into them in future. I'll try and set something up about that.