It came to our attention that Sound Forge 9 still has problems with multi-channel plug-in handling. However, one of our users has reported that this problem can be resolved by instantiating plug-in in SF9's built-in "plug-in chainer". Also make sure you have upgraded to the latest SoundForge v9.0e which does not exhibit this problem.
Most probably this is a problem in SoundForge 8 that cannot deal with multi-channel capable plug-ins correctly (so, this problem happens with all other multi-channel plug-ins). You may use SoundForge 9 where this problem was fixed.