debian_pipeline with enable_make_signed_source and enable_upload doesn't upload signed sources/binaries

debian_pipelines with enable_make_signed_source (with appropriate packages in signing_template_names) and enable_upload cause the regular package to be uploaded to the specified repository, but not the template packages and their binaries, see this private pipeline for an example.

I've noticed that there are 4 merge uploads tasks, all of them for the main package, where I'd expect 1 for the main package and 3 each of the template packages. Not sure if that's related.

/cc @cjwatson