System.ServiceModel.InstanceContextMode not found in System.ServiceModel.Primitives (8.0.0, and lower) nuget package

I added the nuget package "system.servicemodel.primitives" (8.0.0). The enum InstanceContextMode is not present there.

The source Code of the enum is here [InstanceContextMode.cs](https://github.com/dotnet/sdk/issues/src/System.ServiceModel.Primitives/src/System/ServiceModel/InstanceContextMode.cs) on git . The decompiled source of the dll in lib folder of the nuget package looks the same (everything right).

I found out, that the file used in the project is in ref folder not in lib. The file sizes differ extremely:
In ref folder: 113KB
In lib folder: 2.28MB