runtime: DllNotFoundException after successful manual dlopen on MacOS

I am trying to embed Python 3.6 into my cross-platform C# app.

Since I want the app to rely on an existing Python installation, I let the user pick the Python interpreter, then preload its the dynamic library with LoadLibrary/dlopen, and rely on DllImport to use the preloaded library.

This approach works no problem on Windows and Linux (though in the later case I only tried with the library actually being in the load path), however on MacOS I get Unhandled Exception: System.DllNotFoundException: Unable to load shared library 'python3.6m' or one of its dependencies. In order to help diagnose loading problems, consider setting the DYLD_PRINT_LIBRARIES environment variable: dlopen(libpython3.6m, 1): image not found

Here’s the repro example:

using System;
using System.Runtime.InteropServices;

namespace DynPreload
{
    class Program
    {
        static void Main(string[] args)
        {
            const string pythonLib = "/usr/local/opt/python36/Frameworks/Python.framework/Versions/3.6/lib/python3.6/config-3.6m-darwin/libpython3.6m.dylib";
            if (dlopen(pythonLib, DLOpenFlags.RTLD_NOW) == IntPtr.Zero) throw new Exception();
            Console.WriteLine("Preloaded!");
            Py_SetProgramName("42"); // <- this throws DllNotFoundException
        }

        enum DLOpenFlags {
            RTLD_NOW = 2,
        }
        [DllImport("dl")]
        static extern IntPtr dlopen(string path, DLOpenFlags flag);
        [DllImport("python3.6m")]
        static extern void Py_SetProgramName(string name);
    }
}

(to get Python 3.6 installed into the specified folder using brew, run brew install sashkab/python/python36)

This happens despite dlopen successfully preloading libpython3.6m.dylib in the second line.

TargetFramework is netcoreapp2.0