item_id is the ID of the item, not the record ID:

from VuFind documentation: "item_id - the item (as opposed to bibliographic)
identifier (optional)"

-> when parsing JSON take $item["id"] as item_id

module/VuFind/src/VuFind/ILS/Driver/DAIA.php

@@ -530,7 +530,7 @@ class DAIA extends AbstractBase implements
             foreach ($daiaArray["item"] as $item) {
                 $result_item = [];
                 $result_item["id"] = $id;
-                $result_item["item_id"] = $id;
+                $result_item["item_id"] = $item["id"];
                 $result_item["ilslink"] = $doc_href;
                 $number++; // count items
                 $result_item["number"] = $number;