Honestly not sure if it is a bug or intended. My guess is intended. For example if you put a silencer on an M4 and use STANAG rounds you get the same zombie aggro. If you use STANAG SD rounds it is truly silent and you don't get aggro.
This is the same for sidearms as well. A ailenced UZI can use a number of different magazines. But only using M9SD or MP5SD mags doesn't cause zombie aggro.
It does seem that the silencer should lessen the aggro distance with non-SD ammo.
Yea honestly, if you want to talk realism it is pretty accurate. Having had the pleasure of firing a suppressed M4, it is not all that quiet. Most of the noise comes from the fact that standard ammo is supersonic and as such creates a small sonic boom when fired.