Previous Hydra evaluation doesn't work

When I use Previous Hydra evaluation as an input, it arrives with only subset of jobs in it and I see errors like:

in job ‘build’:
attribute 'my-app' missing, at /nix/store/bhkvbl93hh6vgwqarcr6hks6qclimarc-source/nix/release.nix:15:19

The evaluation should be like:

{
    my-app = {
        tarball = mkDerivation { ... };
    }
    package = mkDerivation { ... };
    source = mkDerivation { ... };
}

Through experiments I've found that it has only package job in it. All others are lost. But they work flawlessly if I pass them individually as Previous Hydra builds.

Reproduce the issue using Previous Hydra evaluation and compare its jobs with individually supplied Previous Hydra builds. Start by tracing how the evaluation input is assembled around nix/release.nix; done means the evaluation preserves jobs such as my-app, package, and source without the reported missing-attribute errors.