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 to AnswerHub just mention UE-59508 in the post.