Following this technical article to connect a Survey123 survey with an unfederated server feature service, How To: Create Survey123 for ArcGIS surveys that connect to non-federated ArcGIS Server services
When the feature service is added to ArcGIS Online, you have to embed the credentials to the layer.
This disables the credential pop-up dialogue, and subsequently makes every submitted survey appear to be created by the same user if using editor tracking.
If Survey123 would forward credential requests and send a credential pop-up that would help with editor tracking in unfederated servers.
I'm sure Portal with federated servers completely eradicates this problem, but not many people are there yet.