Sorry about the confusion! From what you’ve said, your character has the Sorcerer class-- it won’t say “mastered” unless you are. You need to press the “unlock mastery trees” button before you can allocate points into the Mastery Trees. This button is located at the top of your Mage tree. We plan to make this process easier to understand.
The bug where you’re seeing Acolyte masteries is known. It’s just a display error, but you’re right that it should show Mage masteries. You can’t change your mastery after picking it, so you’re not missing any functionality.