I will try on Windows later on today. I tested with OSX earlier and didn't have any issues. For what it's worth, do you have 4 channel output from your plugin when you open the Cabbage patcher?
I'm seeing the same behaviour on Windows. I'll take a look. It's strange because it works fine in the Cabbage patcher, which is basically a VST host in itself. And strange too that we get different behaviour on OSX. Leave it with me.