I mentioned in my earlier post that I had experienced a couple of issues. The second one is in relation to changing parameter values from the host, rather than the plugin UI.
I've screengrabbed this using the Juce audio plugin demo and host (showing the "Show All Parameters" window), so it's clear what I'm talking about:
You can see that changing parameters in the host UI has no effect, which is the same experience I'm having with my plugin when running as VST3. Again, I've tried to debug through the Juce code, but I'm afraid I didn't form any great conclusions about what's going wrong with it.