I've reported an issue in composer/satis (composer/satis#715), but as far as I can tell the issue originates here. Composer's ArchiveManager::archive() doesn't seem to preserve the file modification timestamps from the source when creating an archive.

In Satis, when a package is re-archived, the unzipped files have a modified time of 1 January 1980 at 00:00. Its important (for caching purposes at least) that the file modified timestamps match the timestamp of the tag/release.