Hello Mikael, neilwilkes,
what you described is not bug but feature.
I replicated your steps blindly with all compressors what I have.
Than I realized that you have Feed Back button enabled. Which I assume bypass side-chain changes (or eliminate by design?) more about it on Rupert's site http://rupertneve.com/products/portico-543-mono-compressor-limiter/. When FB is off compressor reacts as you expect.
Shame on Steinberg that nobody take care on you! So good plugins with so bad support.