Please let me give additional information.Using "appstreamcli validate foobar.xml" on my local machine do not give me a warning about the length of that field. That confuses me much more. Which check command is used on the Debian Build Servers to generate that warning message?
I opened a ticket at the AppStream project about their specificaton of that field. Currently their docu do not tell about a length limit.
<https://github.com/ximion/appstream/issues/648>
