Yes it does... If you craft 3 items at once, when you exit you backpack it'll say +30, +30, +30, one after the other.
It doesn't show the +30 on screen if you craft multiple items at once but if you do the math you do gain all the parts.
However, if I do remember correctly, when the game first came out, crafting multiple items in your backpack only gave you +30 parts (I'm pretty sure). So you could craft 5 items and get the same parts as crafting 1 item, but they fixed it.
It was never messed up, people just thought it did because they didn't see the points being actively given, the points gained are given to you as they are achieved. Meaning if you're in another HUD such as your backpack or the stats screen, the gameplay hud containing your parts will be temporarily out of sight, so you won't see yourself gaining points.
-- It's a common fact that anyone who does better than you is a lagger.