Dragon Age Origins / Arcane warrior question / I still wear an armor after disactivating the magic ?

I have an arcane warrior as well and you are not experiencing a glitch or cheat. The reason you are having a mana problem is because your willpower (or whatever stat raises your mana pool) is not high enough. You should dump all of your points into Magic and Willpower along with Constitution so you can have the health to fight. Also, if you are a heavy spell user, arcane warrior may not be a way to go for you. The point of going "invisible" is do you can replace your Magic stat with your Strength stat making your physical attacks more useful and that's why it is good to use a sword instead of a staff. That is also why you are still able to wear the armor because it sees your Magic as your Strength. Arcane Warrior is very effective if you use him/her right. I have an Arcane Warrior/Spirit Healer so not only do I deal a lot of damage I can keep my party alive also. I've also heard the arcane warrior/blood mage is a pretty good set up too. You just have to make sure you have high constitution because they use health instead of mana. So in short, an Arcane Warrior is more of a warrior with the ability to use magic as support but the main focus is physical attacks.