I'm not aware of any spatial reference issues that would allow you to edit in Map Viewer Classic but not the new Map Viewer.
The basemap sets the spatial reference of the Web Map and is what data will be collected in e.g. esri is WGS84 web mercator. If the layer is different e.g. state plane, the data will be projected from WGS84 to State plane with default transformations applied unless one is specified in the service definition.
Generally, the best would be to have your basemap and all layers in the same spatial reference. Alternatively, consider any projections and transformations that may be applied between the external GPS provider (location profile), map, layer, and data source if using ArcGIS Server.
But the issue described does not sound like it is related to projections. How are you publishing the data? Do you get the issue if you add the data to a new map with full editing control? Do you get any messages? Can you edit other layers?
Please don't forget the kudos 🙂
