The .NET libmongocrypt wrapper fails to load mongocrypt.dll when running on .NET Framework. This is due to a difference in how libraries are loaded: for .NET Core applications, the wrapper library is loaded from ~/.nuget/packages/MongoDB.Crypt-[version], which contains the folders that contain the native libraries, whereas for .NET Framework, the wrapper is loaded from the copy of the DLL that's bundled with the built  application.