Fair enough!

Would you say the best work-around is the post-relate field type? User just has to add new posts first before they can be chosen to relate to another post, right?

Yes, that’s exactly what I’ve tried.

I’ve now reproduced this issue again on a simpler project. Again – the first post is saved perfectly, but when using the “add more” feature, the second post doesn’t get saved at all.

This field is in a meta box on custom_post_type_1:

piklist('field', [
	'type' =>   'group',
	'scope' =>  'post',
	'label' =>  'Add Staff',
	'relate' => ['scope' => 'post'],
	'add_more' => true,
	'fields' => [
		['type' => 'hidden', 'field' => 'post_type', 'value' => 'custom_post_type_2', 'scope' => 'post'],
		['type' => 'hidden', 'field' => 'post_status', 'value' => 'publish'],
		[
			'type' => 'text',
			'scope' => 'post',
			'field' => 'post_title',
			'label' => 'Name',
			'columns' => 6
		]
	]
]);

If I add an instance of custom_post_type_2 using this form, the data is saved and the new post is related properly. (Interestingly, this data is also preloaded when I refresh this form after saving.)

If I add a second instance of custom_post_type_2, nothing is saved at all to the database when I submit the form.

OK, I found this in the Demo:

add_filter('piklist_post_submit_meta_box', 'piklist_demo_post_submit_meta_box', 10, 3);

I’m not sure if there’s any documentation on that hook? – but it seems to accommodate what I’ll need 🙂