FYI, the issue is still there with Dorico 5 and JUCE 7. It seems also very similar to Wavelab 10 crash on quit inside VST3 wrapper - #6 by pflugshaupt . I’m trying the following workaround which seems to avoid the crash:
add this method to VstComSmartPtr:
void forget() { source = nullptr; }
and call it in the JucePluginFactory destructor:
host.forget();
I guess the crash happens because Dorico destroys the host pointer before the JucePluginFactory is destroyed