Rendering a form using \Drupal::service('entity.form_builder')->getForm($entity, 'default') is returning a plain form, before hook_form_alter
Ensure that your implementation of hook_form_alter() runs at the right time via hook_module_implements_alter().


  This hook is invoked during
  \Drupal::moduleHandler()-&gt;getImplementations(). A module may implement
  this hook in order to reorder the implementing modules, which are
  otherwise ordered by the module's system weight.


But pay attention when using possible variations of hook_form_alter().


  Note that hooks invoked using \Drupal::moduleHandler-&gt;alter() can have
  multiple variations(such as hook_form_alter() and
  hook_form_FORM_ID_alter()). \Drupal::moduleHandler-&gt;alter() will call
  all such variants defined by a single module in turn. For the purposes
  of hook_module_implements_alter(), these variants are treated as a
  single hook. Thus, to ensure that your implementation of
  hook_form_FORM_ID_alter() is called at the right time, you will have
  to change the order of hook_form_alter() implementation in
  hook_module_implements_alter().


See? Right now you are checking for if ($hook == 'form_FORM_ID_alter') which won't work. Fix it by replacing it with if ($hook == 'form_alter').



If that doesn't work try to change your module's weight via module_set_weight().