Restore_cache with multiple keys restores wrong cache

I am using a backup cache key key incase the checksum doesn’t work:
https://circleci.com/docs/2.0/configuration-reference/#example-2

Between my two steps, the second always seems to restore from the non-checksummed cache.
https://circleci.com/gh/wellcometrust/wellcomecollection.org/5510
https://circleci.com/gh/wellcometrust/wellcomecollection.org/5512 <= second restore cache

I thought this wasn’t to be expected but I am obviously missing something and don’t want to be superstitious about it.