Would it be possible for you to update to v1.4? It looks like this exact set of commands now works.
My hypothesis is that newer versions of Pkg are now able to look into B’s manifest to find where A comes from… Of course, you’ll have to commit all Manifest.toml files into your git repository, but one advantage of your current setup (all local packages in the same repo) is that relative paths stored in the manifests remain valid wherever that repository is cloned.