simplestreams index file generation doesn't append new data

juju metadata generate-tools creates simplestreams metadata for tools tarballs.
When run more than once, it is supposed to append any new metadata to the existing metadata.
The recent introduction of explicit support for released/proposed/testing/devel tools has broken this.
When the generate utility is run a second time with a different stream, the entire content for a previous (different) stream is replaced in the index file, instead of a new content block being created. Running a second time with the same stream still works as expected though.