The changes in dotnet/sdk#43627 caused the vertical manifest, which previously had a well-known name of VerticalManifest.xml, to have a name that is based on the job name which produced that file. This causes issues when consuming the artifacts tarball as the previously source built artifacts for source build. That relies on knowing the file name to retrieve: https://github.com/dotnet/sdk/blob/77d763f12701c985bad0a6ed6bbba4c9e7b1c8cb/src/SourceBuild/content/repo-projects/Directory.Build.targets#L303.

By not having the vertical manifest file, the previous package versions don't get populated, preventing the build from being able to use any of the packages from the previously source built artifacts.