Post-fader send to surround bus works as pre-fader

I'm currently working on a 40 tracks project on Cakewalk Sonar x3 64bit  (Windows 7 Ultimate x64)
i already know the difference between pre / post fader send.
What I found today is that when I setup a post send from a track to a stereo bus I lower the channel volume fader and the signal on that bus go down.
but when i set a post-fader send to a surround bus it doesn't care about the value of the volume fader, even if i set it to -infinite ... the signal on the surround bus is full volume.
the only way to modify the level send of to the surround bus when the volume fader changes is switching on the Automation Write button, so it gives me the fact that there's a way to make that when the track volume fader change ... affect the level send to a surround bus ... as it does if i send it to a stereo bus...
but im not getting the key to fix it ...
any idea...?
is a bug..?
Sonar documentation teaches the difference between pre and post fader bus says it's the same if after choose pre/post fader you send to to a stereo / surround bus or a hardware output so....
any help?
Thank you!
* in the image, the volume fader is at -15dB and the bus "ambient" are also at -15dB but the level on the surround bus "Room" are louder, still like 0dB.