Problem/Motivation
This is what I had to do to get AjaxResponses in a multistep form for entity_embed for media.
$output = drupal_render($rebuild_form);
drupal_process_attached($rebuild_form);Just calling drupal_render here should have been sufficient, requiring black magic knowledge.
Proposed resolution
TBD
Remaining tasks
TBD
User interface changes
None
API changes
TBD