The build.extra field in Koji was not intended to hold huge amounts of data. The modulemd_str value can be quite large and causes problems when Koji is configured to use the protonmsg plugin.
modulemd_str
See also: https://pagure.io/koji/issue/2017
This issue has been migrated to Fedora Forge: https://forge.fedoraproject.org/koji/mbs/issues/1592
Please continue any further discussion there.