With the new "liege levy" system (which I like BTW, less micromanagement is worth the loss in flexibility in my opinion), raised vassal levies remain garrisoned. Also, when your liege raise your troops, I don't think you lose anything, and you can actually still raise them on your own (only the amount of troops guaranteed through CA is unavailable to you, whether they are raised or not)! I don't know if it's WAD or a bug, it's hardly gamebreaking, but it's a tad silly and I'd like to know if there's any plan to "fix" that in any upcoming patch or hotfix.