If a recipe downloads a bare file in SRC_URI (i.e. not a tarball), and doesn't download a git repo, the source date epoch can be inconsistent. This can particularly happen if the file ends up in a pre-mirror. The reason is that the reproducible code falls to get_source_date_epoch_from_youngest_file(), and this date will likely be the date of the downloaded file. Note that this generally doesn't affect file:// SRC_URI values, because their mtime is fixed.

The fix is probably that get_source_date_epoch_from_youngest_file should detect single file downloads (e.g. non-tarball files) and skip itself, allowing SOURCE_DATE_EPOCH_FALLBACK to be used instead. Alternatively, get_source_date_epoch_from_youngest_file might be able to be completely removed, although it does provide value for the common case of downloading a tarball.