This was reported to me by @puiterwijk , who has worked around the issue here
https://github.com/fedora-iot/rpm-head-signing/pull/61/files
In rpm_hdr_size, Koji pads the header length to a multiple of 8 bytes. However, it seems that this is only appropriate to do for signature headers.
rpm_hdr_size
Fix forthcoming
The Maximum RPM book alludes to the padding thusly: https://ftp.osuosl.org/pub/rpm/max-rpm/s1-rpm-file-format-rpm-file-format.html
There are four null bytes following the last data item in order to round the size out so that it ends on an 8-byte boundary.
The rpm.org docs say: https://rpm-software-management.github.io/rpm/manual/format.html
the Signature section is padded to a multiple of 8 bytes
Neither indicates similar padding for the main header
Metadata Update from @mikem: - Custom field Size adjusted to None
Metadata Update from @tkopecek: - Issue set to the milestone: 1.33 - Issue tagged with: testing-ready
Metadata Update from @mfilip: - Issue tagged with: testing-done
Commit 72c6495a fixes this issue
This issue has been migrated to Fedora Forge: https://forge.fedoraproject.org/koji/koji/issues/3689
Please continue any further discussion there.