I've made an example that is pretty bare-bones, hopefully this helps out.
In this example there are two date-metaboxes, only difference is that the second has clone=true (and clone_as_multiple=true which does not change anything). With clone=true I get the following error:
Warning: DateTime::createFromFormat() expects parameter 2 to be string, array given in /Users/ronald/Sites/ronald.local:5757/wp-content/plugins/comsi-ce/vendor/meta-box/meta-box/inc/fields/datetime.php on line 175
If I change that line 175 and add a timestamp to the DateTime::createFromFormat(), the error disappears and it works as expected.
[..], $meta . ' 00:00:00' );
But again, this is only the case when clone=true (second example).