I've created a simple form in Drupal 8. When I create a route towards it, it's working fine. The values I submit are dumped in the submitForm function, but as soon as I render the form within a block the $form_state variable is always empty in submitForm.

I've done this a hundred times and it works on every site, but not on this Drupal 8.9 install, so I am wondering if something might have changed.

Here is my form:

    <?php
    /**
     * @file
     * Contains \Drupal\example_filters\Form\FilterForm.
     */
    namespace Drupal\example_filters\Form;
    
    use Drupal\Core\Form\FormBase;
    use Drupal\Core\Form\FormStateInterface;
    use Drupal\Core\Routing\TrustedRedirectResponse;
    
    class FilterForm extends FormBase {
      /**
       * {@inheritdoc}
       */
      public function getFormId() {
        return 'example_filter_form';
      }
    
      /**
       * Build the form
       *
       * @param array $form
       * @param FormStateInterface $form_state
       * @return array
       */
      public function buildForm(array $form, FormStateInterface $form_state) {
        $form['#tree'] = FALSE;
    
        $form['name'] = array(
          '#type' => 'textfield',
          '#title' => t('Name'),
        );
    
        $form['city'] = array(
          '#type' => 'textfield',
          '#title' => t('City'),
        );
        $form['#cache'] = [
          'max-age' => 0
        ];
        $form['actions'] = [
          '#type' => 'actions',
        ];
        $form['actions']['submit'] = [
          '#type' => 'submit',
          '#value' => $this->t('Filter'),
        ];
    
        return $form;
      }
    
      /**
       * Submit handler
       *
       * @param array $form
       * @param FormStateInterface $form_state
       */
      public function submitForm(array &$form, FormStateInterface $form_state) {
        $values = $form_state->getValues();
    	dump($values);
      }
    }


and my block code:

    <?php
    /**
     * @file
     * Contains \Drupal\example_filters\Plugin\Block\FilterBlock.
     */
    
    namespace Drupal\example_filters\Plugin\Block;
    
    use Drupal\Core\Block\BlockBase;
    
    /**
     * Provides a 'Filter' block.
     *
     * @Block(
     *  id = "search_filter_block",
     *  admin_label = @Translation("Filter block")
     * )
     */
    class FilterBlock extends BlockBase {
    
      public function build() {
        return \Drupal::formBuilder()->getForm('Drupal\example_filters\Form\FilterForm');
      }
    }