Arkania/ArkCORE — comment on commit 0c8971a91e02c0f55f059dc4008ceead9df71c22
File:  line 0
By MystRebirth:

Because The hidden spell cast is it's own packet. On top of that, if you read the changes, both a hidden and shown spell cast can be present with some quests in Cataclysm, therefore, both are required. You may learn a spell or have a spell cast on you at the end of a quest, but the same quest could also have a phase changing spell behind it. Doing negatives is just a hacky fix and does not even come close to fixing this properly.