Hi Chris,
My mistake, I thought you were talking about the output being downmixed.
From the looks of it, any multi-channeled audio file above 7 channels will get auto-downmixed if it does not have the exact number of channels FMOD is expecting. 7.1 (8 channels) and 7.1.4 (12 channels) work fine in our testing, but 9, 10, and 11 channel files have this downmix issue you are experiencing.
I have raised a task in our tracker to investigate this further. Thanks for bringing it to our attention.
In the meantime, if you make the channel count 7, 8, or 12 for your file it should work fine.
Thanks,
Richard