The usage of static properties to track the calling state of those callbacks is the same solution than used in #11853 (for the same reason, as the new implementation based on lazy objects in 3.4.x had to solve the same issue in the testsuite)

Here’s where I’m at about the remaining failure:
The entity ends up in this intermediate buggy state because of a call to __setInitialized in the ORM.
This call disables lazy initialization without actually setting the values of the lazy properties.
One difference between the native implementation and that of var-exporter is precisely about default values: the native one initializes with default values, while var-exporter leaves them uninitialized.
I patched the implementation in lazyGhostTrait, but it causes another error when I run the tests.
Something to dig into. I’ll need to take a break — not sure if I’ll be able to pick it up again before I leave for holidays.
I’m wondering if the ORM shouldn't set the default values of all lazy properties on its own after a call to __setInitialized. That would make some sense. But I don’t yet understand the new error, so we should see if that gives any other ideas also.

    #[Ignore]
    private function setLazyObjectAsInitialized(bool $initialized): void
    {
        if (!$state = $this->lazyObjectState ?? null) {
            return;
        }

        if (!$initialized) {
            $state->status = LazyObjectState::STATUS_UNINITIALIZED_FULL;

            return;
        }
        
        $initializer = $state->initializer;
        $state->initializer = static fn () => null;

        try {
            $state->initialize($this, '', null);
        } finally {
            $state->initializer = $initializer;
        }
    }

I figured it out:

The behavior of lazy-ghost-trait is desired on the side of Doctrine's ReflectionProperty since this is how doctrine/persistence can set a property without triggering lazy-initialization.

But on the UnitOfWork side, we do need to set all props to their default value.

Here is a patch that does so:

--- a/src/UnitOfWork.php
+++ b/src/UnitOfWork.php
@@ -49,6 +49,7 @@ use Doctrine\Persistence\PropertyChangedListener;
 use Exception;
 use InvalidArgumentException;
 use RuntimeException;
+use Symfony\Component\VarExporter\Hydrator;
 use UnexpectedValueException;
 
 use function array_chunk;
@@ -2944,6 +2945,7 @@ EXCEPTION
 
             if ($this->isUninitializedObject($entity)) {
                 $entity->__setInitialized(true);
+                Hydrator::hydrate($entity, (array) $class->reflClass->newInstanceWithoutConstructor());
             } else {
                 if (
                     ! isset($hints[Query::HINT_REFRESH])

Thanks @stof !

Thanks @stof