roslyn: Could not load System.Collections.Immutable

Version Used: Microsoft.CodeAnalysis.CSharp 4.3.0 for .NET 6

Steps to Reproduce:

1. Get compilation var c = (CSharpCompilation)context.Compilation;
2. Get globalnamespace and cast him as INamespaceOrTypeSymbol var n = (INamespaceOrTypeSymbol)c.Assembly.GlobalNamespace;
3. Get members var members = n.GetMembers();

Expected Behavior: No error.

Actual Behavior: Failed to generate source.

The error appeared only in version 4.3.0. Connecting nuget System.Collections.Immutable directly does not help. If you comment out this line, then there will be no error:

//var members = n.GetMembers();

Example code for reproduce error Generator code:

using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;

namespace SourceGenerator
{
    [Generator]
    public class HelloSourceGenerator : ISourceGenerator
    {
        public void Execute(GeneratorExecutionContext context)
        {
            // Find the main method
            var mainMethod = context.Compilation.GetEntryPoint(context.CancellationToken);

            var c = (CSharpCompilation)context.Compilation;
            var n = (INamespaceOrTypeSymbol)c.Assembly.GlobalNamespace;
            var members = n.GetMembers();

            // Build up the source code
            string source = $@"// <auto-generated/>
using System;

namespace {mainMethod.ContainingNamespace.ToDisplayString()}
{{
    public static partial class {mainMethod.ContainingType.Name}
    {{
        static partial void HelloFrom(string name) =>
            Console.WriteLine($""Generator says: Hi from '{{name}}'"");
    }}
}}
";
            var typeName = mainMethod.ContainingType.Name;

            // Add the source code to the compilation
            context.AddSource($"{typeName}.g.cs", source);
        }

        public void Initialize(GeneratorInitializationContext context)
        {
            // No initialization required for this one
        }
    }
}