Get category name from custom loop and echo it once

Here is an idea. Note, this will only work if all posts are in the desired category and if all posts have only category

The posts are returned in an array which you can access through

$currentissue->posts 

With this in mind, you can get the first post ID as follows

$currentissue->posts[0]->ID

Now, you can add that to a variable

$id = $currentissue->posts[0]->ID    

You can now use get_the_category to retrieve the category this post belongs to. Remember, this will be the category that all posts belongs to according to your question

Something like this would work just ouside your loop, just inside your if statement

$category = get_the_category( $id ); 
echo $category[0]->cat_name;