flatten for content files not working

[MattiasEppler]

Referring to the closed issue

#6213

  <ItemGroup>
    <Content Include="native\*">
      <PackageCopyToOutput>true</PackageCopyToOutput>
      <PackageFlatten>true</PackageFlatten>
    </Content>
  </ItemGroup>

PackageFlatten works only if the consuming project is a .net Framework Project. If you use the new core format csproject Fille (sdk) the files are not flatten in the output directory.

[donnie-msft]

Thanks for reporting. Please fill out the Issue template here: https://github.com/NuGet/Home/issues/new
This helps in understanding your product versions and scenario.

Documentation still suggests it should be working: https://docs.microsoft.com/en-us/nuget/reference/msbuild-targets#including-content-in-a-package
/cc @zkat

[MattiasEppler]

Details about Problem

NuGet product used: Visual Studio UI

NuGet version 5.7.0.6726

VS version: 2019

OS version: Windows 10 Pro 1903 / 18362.30

Visual Studio Repros added

NugetPackeSample.zip
Consuming.zip

NugetPackageSample -> VS Solution to create a Nuget Package with content files
Consuming -> VS Solution with two Console Projects (.Net Core and .Net Framwork Format) with reference to the nugetpackage

Detailed repro steps so we can see the same problem

1. The content files in the nuget package are in the subfolders content and contentfiles.
2. The content files in the consuming project should be copied flatten to the outputdirectory after build
3. In the project with the .net framework format the files are copied flatten to the output directory
4. In the project with the .net core format the files are also copied to the output directory. But not flatten.

[MattiasEppler]

Triage:NeedsRepro can be removed. Repro has already been added.

[quintonn]

I am having this same problem.
Is this a bug only with .net core projects because I've read it works with .net Framework projects and in fact I am able to achieve the desired outcome with some of my .net framework projects, but not using .net core (5)?