Unexpected behaviour in foreach while looping and unsetting in ArrayObject. An item is ignored
From my understanding , it is considered a bad practice to modify an array while looping through it, and the proper way to do it would be using array_filter.
Since you have an ArrayObject, one solution would be to export it to an array, filter it using array_filter and create a new ArrayObject from the filtered array.
See also here : Filter ArrayObject (PHP)
Probably this behavior is due to the fact that loops are handled differently in php7. As mentioned here: http://php.net/manual/en/control-structures.foreach.php in php5 foreach uses an internal array pointer in contrast to php7.