If you set the Voice Detection Mode in TeamSpeak 5 to Volume Gate in the Audio Settings and set a value with the slider (e.g. 5dB), it happens to me that although my background noises are quieter and this green slider also indicates this, the sounds are still transmitted.
But there must be a noise which is loud enough to trigger VAD. This is the only way to trigegr it in volume Gate.
The indicator only updates ~200ms and it is possible to trigger VAD without seeing it. Can be tested by saying “Hi” quick enough.
So we do not see this as a bug.
You may use Hybrid mode to filter more noise which is not your Voice.