See line 19, where the problem lies.

iterator.php

if( $this->request->query( 'fmt' ) == 'items' ) :
	$invoices	= collection( $invoices )->map( function( $invoice, $key ) use ( $columns ) {
		// Iterated through line items, rather than invoices:
		$lis	= collection( $invoice->line_items )->map( function( $line_item, $k ) use ( $invoice, $columns ) {
			$line	= [];
			foreach( $columns as $ckey=>$meta ) :
				// For line item lists:
				if( preg_match( '/line_item/i', $meta ) ) :
					$meta		= preg_replace( '/line_item\./i', '', $meta );
					$line[]		= $line_item->{$meta};
				// For invoice lists:
				else :
					$line[]		= $invoice->{$meta};
				endif;
			endforeach;
			return $line;
		} )->toArray();
		return $lis;
	} )->combine(
		'{n}',
		function ( $line ) { return $line; }
	)->toArray();
	die( pr( $invoices ) );
endif;