I just had my roommate test this. I'm still not convinced that this is a bug.

Here is what we observed: When you ADS directly after hip firing 2-5 rounds into your clip, the amount of recoil is comparable to if you had pure ADS fired 20 to 30 rounds of your clip. The important thing is though, when you ADS directly after hip firing 30 rounds or greater, your recoil increases dramatically when compared to when you just hip fired 2-5 rounds.

This is because the "recoil ceiling" is much high for hip fire than it is for ADS fire. This is why you see two "different" recoil patterns. I put the word "different" in quotes because the recoil is exactly the same, the only variable that changes is the magnitude of recoil.

So here are the rules dictating recoil:

The longer you fire your weapon, the greater your weapon builds recoil.
Hip firing your weapon builds recoil faster than ADS firing, and with a higher potential for maximum recoil.
ADS firing your weapon builds recoil slower than hip firing, and with a lower potential for maximum recoil.

It's actually kind of cool because when you ADS after hip firing half your clip, you can actually intimately see how your gun is behaving besides the widening reticle on your HUD. This is because the recoil accumulated by hip fire will directly transfer over to your ADS fire mode should you choose to not cease fire before aiming down your sight.