Work only in the Default model in any DGN file where you are creating civil data like geometries or corridors.
Do not create new models for civil things with different names. ORD needs to be able to create and manage the Default-3D model. Using models with names other than Default for civil stuff may interfere with ORD's ability to create the managed model and correctly attach it as a reference. Just don't go there.
Create other DGN FILES from seed (with their own Default model) when you need to separate your data. Then attach it as a reference - Default to Default. Use the sequential file naming like, GEOM_01, GEOM_02, etc. along with a good description. Use container files to reference together civil data by type.
Review the information in the OpenRoads Designer - ODOT training manual, Appendix C: Managed Models and Appendix D-1: Container files for Referencing