Export a surround project

Hi everyone,

I'm working in surround and would like to export the mix in split mono. But the result is that 2 channels (.L, .C) are a bit longer (and bigger in size) that the 4 other channels ! (this is a big problem because those 6 tracks aren't recognize as a surround mix by other software like ProTools).

I used to work with Sonar 6 and thought this version was not able to do surround mix properly. So I upgrade to Sonar 8 and the result is the same !!!

As anyone an idea ? thanks a lot in advance.