The "Edit on Bynder" field that can be drag/dropped/re-ordered on the "Manage form display" tab for Bynder media types doesn't actually work unless the form is the default /media/ID/edit form.
This is due to the following:
/**
* Implements hook_form_alter().
*/
function bynder_form_alter(&$form, FormStateInterface $form_state, $form_id) {
if ('media_bynder_edit_form' == $form_id) {
Different form modes have a different form ID.
Always add the element if $form_state->getObject() is an instance of MediaForm.
Active
4.0
User interface
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Ran into this same issue today and was going to create the issue, so I'm +1 on this one. Overall I find this module confusing in that, it provides a media entity source of Bynder, but also provides (in a strange config manner) a single bundle for that config.