The xz format supports concatenated files, images are sometimes created and shipped with it to increase compression.
https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1742744 is the source bug for this issue.
59dda97f7 support LZMA concatenated files
src/import/import-compress.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
Upstream: github.com