MSBuild version 15 introduced the concept of "Directory.Build.props" files. From the docs:

• Directory.Build.props is a user-defined file that provides customizations to projects under a directory. This file is automatically imported from Microsoft.Common.props unless the property ImportDirectoryBuildTargets is set to false.

What is not stated here is that only one Directory.Build.props will be imported automatically. Imagine your project exists in the directory C:\repo\src\foo\foo.csproj and there exists a file in both C:\repo\src\Directory.Build.props and C:\repo\Directory.Build.props then only C:\repo\src\Directory.Build.props will be automatically included when building C:\repo\src\foo\foo.csproj. If you would like C:\repo\Directory.Build.props to be included as well, then C:\repo\src\Directory.Build.props will have to include C:\repo\Directory.Build.props like so:

<Project>
    <Import Project="$([MSBuild]::GetPathOfFileAbove('Directory.Build.props', '$(MSBuildThisFileDirectory)../'))" />
</Project>

This is documented here.