.Net 8 Project Reference Specify .net Version of Multi-Targeted Transitive Project Dependency

Question

I have a .net6 sdk project that I need to upgrade to .net8 for some new functionality. It has some shared project libraries with other projects that are still .net6.

Project A includes Project B which includes Project C. I need A and C to be .net8, but B and C still need to support .net6 projects. So I tried multi-targeting B and C to .net8 and .net6 and upgrading A to .net8. But when I run project A it's reference to C is still the .net6 version.

B and C are outputting dlls for both net6 and net8, but I can't get A to pull in the new versions.

Project A csproj

  <Project Sdk="Microsoft.NET.Sdk.Web">
    <PropertyGroup>
      <TargetFramework>net8.0</TargetFramework>
      <AssemblyName>A</AssemblyName>
    </PropertyGroup>
    <ItemGroup>
      <ProjectReference Include="B.csproj" AdditionalProperties="TargetFramework=net8.0" />
    </ItemGroup>
  </Project>

Project B csproj

  <Project Sdk="Microsoft.NET.Sdk">
    <PropertyGroup>
      <TargetFrameworks>net6.0;net8.0</TargetFrameworks>
      <AssemblyName>B</AssemblyName>
    </PropertyGroup>
    <ItemGroup>
      <ProjectReference Include="C.csproj" />
    </ItemGroup>
  </Project>

Project C csproj

  <Project Sdk="Microsoft.NET.Sdk">
    <PropertyGroup>
      <TargetFrameworks>net6.0;net8.0</TargetFrameworks>
      <AssemblyName>C</AssemblyName>
    </PropertyGroup>
  </Project>

This is a simplified view of what I have. There are many more dependencies, so maybe there's some interference from another project or package. But I'm not sure why A doesn't have the .net8 version of C with this setup. Any ideas on how to achieve this?