AJAX error: node form + list field with multiple options + image field
After additional testing, this seems to be a bug in Drupal core that affects only the select form elements.

Workaround is to change the field type to checkboxes. Everything else can stay the same. Values are then passed properly and everything works OK even with multiple file upload fields added to the same form.

Also the validation and submit handlers work as expected in this case.

I created a ticket for Drupal core here.