DocumentChangeSet not containing original document values if change set is recomputed

[Nilz11]

Bug Report

Seems to be since 10 years unchanged behaviour. Affected versions 2.x and 3.x

Summary

When I recompute the change set, I would expect that it contains the original document data.

So for example I change a field recomputeChangeSet and fetching this change set afterwards contains the updated value as the old value and not the original database value.

I tracked this down to UnitOfWork which does set $this->originalDocumentData[$oid] = $actualData; ~ line 850 depending on the version.

Current behavior

It contains the updated value as the old value and not the original value.

How to reproduce

Short example i made:

class ModifyListener
{
    public function onFlush(OnFlushEventArgs $eventArgs)
    {
        $uow = $eventArgs->getDocumentManager()->getUnitOfWork();

        foreach ($uow->getScheduledDocumentUpdates() as $document) {
            $document->setFirstName('Change in listener');

            $uow->recomputeSingleDocumentChangeSet(
                $eventArgs->getDocumentManager()->getClassMetadata(Customer::class),
                $document
            );
        }
    }
}

class TrackingListener
{
    public function onFlush(OnFlushEventArgs $eventArgs)
    {
        $uow = $eventArgs->getDocumentManager()->getUnitOfWork();

        foreach ($uow->getScheduledDocumentUpdates() as $document) {

            $cs = $uow->getDocumentChangeSet($document);

            var_dump($cs);
        }
    }
}

$customer = (new Customer())
->setFirstName('Insert');

$dm->persist($customer);
$dm->flush();

$dm->getEventManager()->addEventListener('onFlush', new ModifyListener());
$dm->getEventManager()->addEventListener('onFlush', new TrackingListener());

$customer->setFirstName('Update');

$dm->flush();

This outputs using var_dump:

array(1) {
  ["firstName"]=>
  array(2) {
    [0]=>
    string(6) "Update"
    [1]=>
    string(18) "Change in listener"
  }
}

Expected behavior

I would expect that the change set is:

array(1) {
  ["firstName"]=>
  array(2) {
    [0]=>
    string(6) "Insert"
    [1]=>
    string(18) "Change in listener"
  }
}