Form validation breaks after upgrading from 3.4.12 to 3.4.24

[cacharrin]

Symfony version(s) affected: 3.4.24

Description
After upgrading from Symfony 3.4.12 to 3.4.24 the form type throws validation errors.

How to reproduce
I have this entity:

class Lead{
    //more code 

    /**
     * @var string
     *
     * @ORM\Column(type="string", length=60)
     * */
    private $loanPurpose;

    //more code
}

And the corresponding FormType (Note i'm not enforcing any data type)

class LeadType extends AbstractType{

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('loanPurpose')
            //more fields added
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => Lead::class
        ));
    }
}

Then, I have a controller where I get a json content like this:

{
   "loanPurpose":["Purpose1", "Purpose2", "Purpose3"]
}

And I process it like this:

        $data = json_decode($request->getContent(), true);

        $lead = new Lead();
        $form = $this->createForm(LeadType::class, $lead);
        $form->submit($data, true);

        if($form->isValid()) {
            //Convert array to string "Purpose1,Purpose2,Purpose3"
            $lead->setLeanPurpose(implode(",", $lead->getLeanPurpose()));
            $em->persist($lead);
            $em->flush();
       }

This code works perfectly on 3.4.12, but after upgrade to 3.4.24 it throws
leanPurpose: "This value is not valid"

What is going on?

[dmaicher]

This seems to be the same problem as reported in multiple other issues:

#30125
#29841
#29905
...

caused by #29307

[xabbuh]

closing as explained by @dmaicher