When a comment contains a link to an issue (which is relative to the current repo), the link is copied to the new issue, resulting in a link to issues of the new repo (which may not even exist), because they were pointing to issues of the repo where the old issue was.
When a comment contains a link to an issue (which is relative to the current repo), the link is copied to the new issue, resulting in a link to issues of the new repo (which may not even exist), because they were pointing to issues of the repo where the old issue was.