We have a public Survey123 that collects survey points and gathers created by / edited by information appropriately when data is added through the Survey123 app. However, we would like to be able to add features to the same hosted feature service via the Field Maps app. When new points are added via Field Maps, no editor tracking information is collected. The appropriate editing privileges are enabled for the hosted feature layers.
This is essentially by design when working with public feature layers. If the app can reach the layer without authentication, it does not send credentials (e.g. a token) so the server doesn't know who submitted the feature. I'd recommend creating an ArcGIS Idea or logging an official enhancement request if you'd like to see this changed.
Since you're using hosted feature layers, you might be able to create a view layer that has different sharing permissions and use that in Field Maps - this way the editor tracking should work.