How to add a new field in a feed AfterParseBase?

Parsing a CSV file, and the goal is to take an input column and turn it into two Drupal fields. The last 3 characters of the BondId field becomes a new field called TrancheId.

The BondID field looks like

The CSV source field that is not in the input data is mapped

Code to add a field

class CbiFeedAlterSubscriber extends AfterParseBase {

 /**
  * {@inheritdoc}
  */
 public function applies(ParseEvent $event) {
    return $event->getFeed()->getType()->id() === 'bond';
 }

 /**
  * {@inheritdoc}
  */
 protected function alterItem(ItemInterface $item, ParseEvent $event) {
   $old_tranche_id = $item->get('TrancheId');
   if ($old_tranche_id) {
     return;
   }

   // Tranche ID is a new data item derived from BondID
   $bond_id = $item->get('bondid');
   $tranche_id = substr($bond_id, strlen($bond_id) - 3, 3);

   $item->set('TrancheId', $tranche_id);
 }
}

The service is defined

services:
  cbi_feed_alter.feed_alter_event_subscriber:
    class: Drupal\cbi_feed_alter\EventSubscriber\CbiFeedAlterSubscriber
    tags:
      - { name: event_subscriber }

Lower case field names and it works

$item->set('trancheid', $tranche_id);

Similarly for BondID

$item->set('bondid', $tranche_id);

sets the value, mixed case doesn't work.