WorkflowTransitionElement is called twice on a form

Changed to Bug , as per #6

in #2899025-57: Attached field type 'file' not working on WorkflowTransition → , I removed the unique ID.
Please test dev version.

I guess this solves your problem, If not please reopen.

We now have 3 possible options:
Option 1: call transitionElement() directly.

$element['#default_value'] = $transition;
 $element = WorkflowTransitionElement::transitionElement($element, $form_state, $form);
 

Option 2: call WorkflowTransitionForm() directly.

// @todo Use Form/Widget, instead of transitionElement().
$element = WorkflowTransitionForm::createInstance($entity, $field_name, [], $transition);
$element = WorkflowTransitionForm::trimWorkflowTransitionForm($element, $transition);
$element['#parents'] = [];
// Remove langcode to avoid error upon submit.
// InvalidArgumentException: The configuration property langcode.0 doesn't exist.
unset($element['langcode']);

Option 3: call WorkflowDefaultWidget via NodeFormDisplay.

    $element = WorkflowDefaultWidget::createInstance($transition);
// Fetch the element from the widget.
$element = $element[$field_name]['widget'][0];

Automatically closed - issue fixed for 2 weeks with no activity.