Paket v6 cannot be used for .NET source generation

Description

I have a solution that uses source generation and it requires some additional work to make it work with Paket v5. I have my standard projects compile for net5.0, but source generators must be netstandard2.0, so I used the following paket.dependencies:

source https://api.nuget.org/v3/index.json
framework: net5.0

# All NuGet packages for the regular projects go here

group CodeGenerator
  # The source generator should be .NET Standard 2.0
  source https://api.nuget.org/v3/index.json
  framework: netstandard2.0
  nuget Microsoft.CodeAnalysis.Analyzers copy_local:true
  nuget Microsoft.CodeAnalysis.CSharp copy_local:true

Note that both packages for the CodeGenerator havev copy_local:true added, because these packages should have PrivateAssets="all" enabled in the (generated) project-file as described here.

To ensure that the proper packages are used, the paket.references file of my source generator looks like this:

group CodeGenerator
  Microsoft.CodeAnalysis.Analyzers
  Microsoft.CodeAnalysis.CSharp

I upgraded to .NET 6.0 and needed to upgrade Paket to v6 too, but it now fails to properly invoke the source generator. It fails with the following error:

Generator '...' failed to generate source. It will not contribute to the output and compilation errors may occur as a result. Exception was of type 'FileNotFoundException' with message 'Could not load file or assembly 'System.Collections.Immutable, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified.'

When using the traditional approach with including the packages directly in the project-file it works fine again.

Start by reproducing the failure with the shown paket.dependencies and paket.references, then compare Paket v6's generated project-file references with the working direct project-file approach. Trace how the CodeGenerator group handles Microsoft.CodeAnalysis packages and verify that the source generator can load System.Collections.Immutable; done means the .NET 6 source generator builds and runs successfully under Paket v6.