SideChain not working for AAX and Mono/Stereo Configuration

Hey there,

I am developing a plugin which supports either Mono->Mono, Stereo->Stereo and Mono->Stereo channel configurations. Additionally there is a side-chain input.
For the Mono->Mono and Stereo->Stereo case everything works as expected. However, when I load the plugin in ProTools with a Mono->Stereo configuration everything works fine besides the Side-Chain. The received Side-Chain AudioBuffer is always empty even if the side-chain input selected in ProTools clearly has some signal.

In the processBlock function I get references to the signals as follows:

    auto mainInputOutput = getBusBuffer (buffer, false, 0);
    auto sideChainInput  = getBusBuffer (buffer, true, 1);

The constructor of the AudioProcessor looks like:

AudioProcessor (BusesProperties().withInput ("Input", AudioChannelSet::stereo(), true).withOutput ("Output", AudioChannelSet::stereo(), true).withInput ("Sidechain", AudioChannelSet::stereo()))

And the isBusesLayoutSupported:

bool isBusesLayoutSupported (const BusesLayout& layouts) const
{
    
    if (layouts.getMainOutputChannelSet() == AudioChannelSet::mono()
     && layouts.getMainInputChannelSet() == AudioChannelSet::mono())
        return true;
    
    if (layouts.getMainOutputChannelSet() == AudioChannelSet::stereo()
     && layouts.getMainInputChannelSet() == AudioChannelSet::mono())
        return true;
    
    if (layouts.getMainOutputChannelSet() == AudioChannelSet::stereo()
     && layouts.getMainInputChannelSet() == AudioChannelSet::stereo())
        return true;

    return false;

}

The thing I really don’t understand is, that side-chaining works fine in ProTools for all configurations but Mono->Stereo.

Has anybody an idea what’s the problem here?