Personally, I did not observe this bug until iOS 8.1, though it may have been present in 8.0. When the bug occurs, the only way I found to get rid of the problem is to turn VoiceOver off and back on. Toggling the audio ducking option does not resolve the issue.
I will also note that VoiceOver speech is not the only thing that can trigger the issue. I had the bug occur after adjusting the volume with the side buttons, for example. That said, it definitely does not happen if VoiceOver is disabled, so VoiceOver is ultimately causing this to happen.