Can you please perform some debugging to find the issue?
First save a file (I am to assume this works correctly for the first row)
Now edit your code to print the variable before saving like so:
$field_key = "documents";
$value = get_field($field_key, $post_ID);
What is the $value?
I think the issue may be fixed by changing $field_key to actually use the field key, not the field name