To reproduce:

git clone git://anongit.kde.org/ark.git
appstreamcli validate ark/app/org.kde.ark.appdata.xml

It will print:

W - org.kde.ark.appdata.xml:org.kde.ark.desktop:281
Unable to reach remote location
'https://bugs.kde.org/enter_bug.cgi?format=guided&product=ark' - does it exist?

As you can see, the link does exist. This issue affects also other KDE apps btw.