WarningsNotAsErrors ignored for some warnings with /warnaserror option

[rainersigwald]

Given

<Project>
  <PropertyGroup>
    <MSBuildTreatWarningsAsErrors>true</MSBuildTreatWarningsAsErrors>
    <MSBuildWarningsNotAsErrors>ABC123</MSBuildWarningsNotAsErrors>
  </PropertyGroup>

  <Target Name="Dispatch">
    <ItemGroup>
        <P Include="$(MSBuildThisFileFullPath)" AdditionalProperties="Num=1" />
        <P Include="$(MSBuildThisFileFullPath)" AdditionalProperties="Num=2" />
        <P Include="$(MSBuildThisFileFullPath)" AdditionalProperties="Num=3" />
        <P Include="$(MSBuildThisFileFullPath)" AdditionalProperties="Num=4" />
    </ItemGroup>
    <MSBuild Projects="@(P)" BuildInParallel="true" Targets="Warn" />

  </Target>

  <Target Name="Warn">
    <Warning Code="ABC123" Text="Hello from instance $(Num) in pid $([System.Diagnostics.Process]::GetCurrentProcess().Id)" />
    <Exec Command="sleep 1" /><!-- To give worker nodes some time to spin up -->
  </Target>
</Project>

The MSBuildWarningsNotAsErrors is not consistently respected:

❯ dotnet msbuild -m .\foo.csproj /warnaserror

  foo succeeded with 1 warning(s) (0.0s)

    C:\Users\raines\Downloads\foo.csproj(19,5): warning ABC123: Hello from instance 1 in pid 39896

  foo succeeded (0.0s)

    C:\Users\raines\Downloads\foo.csproj(19,5): error ABC123: Hello from instance 4 in pid 3804

  foo succeeded (0.0s)

    C:\Users\raines\Downloads\foo.csproj(19,5): error ABC123: Hello from instance 3 in pid 5872

  foo succeeded (0.0s)

    C:\Users\raines\Downloads\foo.csproj(19,5): error ABC123: Hello from instance 2 in pid 8016


Build failed with 3 error(s) and 1 warning(s) in 0.2s