commentary on the issue from Ghostcrawler.
There were some changes made to how Arcane Barrage, Arcane Missiles, and the Arcane Blast buff interact to prevent a fun little trick that was being used to get the most out of the buff. Second, the Arcane Blast glyph received a flat nerf, losing 2% damage per stack. Unfortunately, along with those changes came a potential bug to that glyph. Arcane Missiles may not be receiving the additional damage bonus at all.
Overall, these aren't terrible nerfs, and probably should've been expected at some point. Arcane Mage is one of the class/spec combos pulling far, far ahead of the competition, so some changes had to happen eventually. Probably the only real issue here is the fact that once again, there should have been some documentation beyond hoping Ghostcrawler bopped onto the forums to confirm or deny.