I’m not certain either. I don’t see any sign that you’ve configured it to be excluded. If you had, you’d see it reflected in your scanner output like
INFO: Indexing files…
INFO: Project configuration:
INFO: Excluded sources: Backup/**
So it must be getting excluded automatically. I wonder, does the .csproj that covers the scope of the Backup directory have test assemblies linked? If so, as in this post you’ll need to add a property to the project to avoid having SonarQube automatically excluding it.