I have a post select field on one of my pages that uses the multiple option.
array(
'id' => 'home_featured_pages',
'name' => __('Featured Pages', 'edit2017'),
'type' => 'post_select',
'use_ajax' => true,
'multiple' => true,
'query' => array(
'post_type' => 'page'
)
)
When I edit the page to begin with it works as it should letting me select as many pages as I want.
After saving the page and coming back to edit it I get each page in its own select2 box.

All the boxes have the same id/name and only the first one gets saved.
I'm using the latest dev-master code.