The bug is caused by this specific commit: https://github.com/WeAreROLI/JUCE/commit/0931fcea18c93998092484c6cc78609148c10c2e
It is not fixed in this commit either: https://github.com/WeAreROLI/JUCE/commit/e55e87ee0ea07e76ffe504401b9f054811b54b83
The bug is present in the Plugin Host example project; if you make a sound into the microphone as the application is starting up it will briefly output that audio. Larger, more complex projects appear to output the audio for longer.