form:
  register:
    title: Registration
    username: Username
    email: Email
    password: Password
    repeat_password: Repeat your password
    accept_terms: Accept %terms%

->add(
    'termsAccepted',
    CheckboxType::class,
    [
        'mapped' => false,
        'constraints' => new IsTrue(),
        'label' => 'form.register.accept_terms',
    ]
)

{{ form_label(form.termsAccepted, 'that custom label is ignored...') }}