The voice engine starts before it is ready. FOnlineVoiceImpl::StartNetworkedVoice is called prior to VoiceEngine->RegisterLocalTalker() in FOnlineVoiceImpl::RegisterLocalTalker(). This leads to the test for IsOwningUser(LocalUserEnum) in FVoiceEngineImpl to fail.
This occurred in 4.18
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-59508 in the post.