note that it does not happen each time. I had one case with a displayed form which worked, while another profile with a submitted form (same form) broke.

Tried to reproduce, without any luck so far.

Looking at code there should always be an id set 🤔

Do you have some hints about the form structure. Does it happen on the root- or a child-form?

I have a CollectionType in this form. The missing id happens for the first element in this collection.

I think this is related to the fact that the fields are added in a PRE_SUBMIT listener. Maybe extractConfiguration is not called properly for dynamic fields, and so the data is incomplete.

For

public function __invoke(FormFactoryInterface $f, Request $r)
{
    $form = $f->createBuilder();
    $form->addEventListener(FormEvents::PRE_SUBMIT, function (FormEvent $e) {
        $e->getForm()->add('coll', CollectionType::class, ['data' => ['foo']]);
    });
    $form->add('submit', SubmitType::class);
    $form = $form->getForm();
    $form->handleRequest($r);

    return $this->render('index.html.twig', ['form' => $form->createView()]);
}

After submit i see id=form_coll_0 for a view variable in the profiler.

Nevertheless, this code looks related:

https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Form/Extension/DataCollector/FormDataCollector.php#L307-L322

The first $output variable is ignored here.

I'm not adding the collection type itself in a PRE_SUBMIT. But the collection type itself adds new children here.

We ran into this bug too. Not sure what's causing it, but this does reproduce the issue:
https://github.com/WubbleWobble/symfony-issue-29291

Fixed in #33999