the pipeline uses various sources to determine the last-modified header of the response:

• the last-modified of the content (or foldermapped content)
• the last-modified of the config (which intern respects config, head.html and global metadata)
• the last-modified of the foldermapped metadata

if the metadata modified for a rendered path contains a last-modified, it will be use as last modified that for that record. i.e. it does not set the last-modified for the response directly. so newer configs or contents will still have newer metadata.