Return the most recently created custom taxonomy

Use get_terms to select that custom taxonomy, ordering by id DESC and returning only one record:

$latest_term = get_terms( 'issuem_issues', array( 'orderby' => 'id', 'order' => 'DESC', 'number' => 1, 'hide_empty' => false );

This presumes that your WP DB is set up normally, so the highest ID number on the custom taxonomy terms will be the most recently added term.

Also, make sure the slug for the custom taxonomy is Issues, not issues. The slug query is case-sensitive.