Unable to unload DLL in .NET Core 3 - dll

Question

I am trying to unload an external assemble but it still sitting in the memory and I can not delete the dll file. Here is my code - What am I doing wrong ?
The uploaded dll is very simple - just 1 class and one method. no dependencies.
I had a look at many samples and see no issue in the code but it still does not work.
Thank you !
class SimpleUnloadableAssemblyLoadContext : AssemblyLoadContext
{
public SimpleUnloadableAssemblyLoadContext( ) : base(isCollectible: true)
{
}
protected override Assembly Load(AssemblyName name)
{
return null;
}
}
public class PluginLoader2
{
[MethodImpl(MethodImplOptions.NoInlining)]
public string getExternalText()
{
string res = "";
var sourcesPath = Path.Combine(Environment.CurrentDirectory, "Plugins");
string[] fileEntries = Directory.GetFiles(sourcesPath);
foreach (string fileName in fileEntries)
{
SimpleUnloadableAssemblyLoadContext context = new SimpleUnloadableAssemblyLoadContext();
WeakReference w_r = new WeakReference(context, trackResurrection: true);
var myAssembly = context.LoadFromAssemblyPath(fileName);
context.Unload();
for (var i = 0; i < 10 && w_r.IsAlive; i++)
{
GC.Collect();
GC.WaitForPendingFinalizers();
}
res += "<br /><br />" + fileName + " live status is " + w_r.IsAlive.ToString();
}
return res;
}
}