How to preserve kint output upon form submit redirection?
Use ksm() instead of kint().

I ran into the same issue when I want to inspect the entity after saving, using drupal's hook_entity_presave and kint

I found this article on how to print variables using kint() which explains that ksm() prints the output inside the drupal page (not before as kint()) does. 

So this example prints the entity without redirecting:

function mymodule_entity_presave(Drupal\Core\Entity\EntityInterface $entity) {
  ksm($entity-&gt;toArray());
}