Symfony version(s) affected: v.3.4.21

Description
Caused by #29307 (/cc @nicolas-grekas)

I've got a MainFormType that contains a collection of SubFormType. SubFormType has a hidden field that holds a JSON-encoded string (generated by JS in my actual use case). SubFormType transforms the string to an array on submit.

Everything has been working fine until 3.4.21 when the form started giving errors. I'm not really sure if this is a BC break or not, but it has definitely caused us some problems.

How to reproduce
I've created a repro branch at https://github.com/simshaun/symfony-standard/tree/form_bc_break_3.4.21

• Run phpunit to confirm errors.
• Downgrade to Symfony 3.4.20 and run phpunit again.