Custom PHP Code in Rules delivers different results than when executed in Devel - SOLVED

Issue:
Custom PHP code when executed in a rule does not behave as expected. In fact, the same code when run in the Devel module's "Execute PHP" block provides the expected results.

Details:
There are three content types: Parent, Child and Actions. Both Parent and Child have node references to Actions. Parent has a node reference to Child.

The goal is to set up a system such that all Child nodes will inherit the Actions referenced within the Parent nodes. Therefore, the rule is triggered to execute custom PHP code on the condition that a Parent node is being saved.

Custom PHP Code

// Get all the associated actions in the parent category
$parent_assoc_action=$node->field_associate_actions;
$parent_assoc_category=$node->field_associate_categories;

// For each child, check if the parent's associated action is already present, update if not
foreach ($parent_assoc_category as $c) {
  $child_cat=node_load($c['nid']);
  $child_assoc_action=$child_cat->field_associate_actions;

  //need to compare arrays child_assoc_action and parent_assoc_action to see if there are any matches
  //make a copy
  $first_array_s = $parent_assoc_action;
  $second_array_s = $child_assoc_action;

  // serialize all sub-arrays
  array_walk($first_array_s,'my_serialize');
  array_walk($second_array_s,'my_serialize');

  // array_diff the serialized versions
  $diff = array_diff($first_array_s,$second_array_s);

  // unserialize the result
  array_walk($diff,'my_unserialize');

  //update child associated actions as needed
  $new_child_assoc_action=array_merge($child_assoc_action,$diff);
  $child_cat->field_associate_actions=$new_child_assoc_action;

  $child_cat=node_submit($child_cat);
  node_save($child_cat);
}

// functions to help process arrays before comparison
function my_serialize(&$arr,$pos){
  $arr = serialize($arr);
}
function my_unserialize(&$arr,$pos){
  $arr = unserialize($arr);
}

This code fails because it results in the referenced Actions of the Parent node being appended to the Actions of the Child regardless of whether the Child has any of the Actions referenced already. This results in duplicates.

However, if I copy the exact same code and run it in Devel's Execute PHP block (the only change being to execute a node_load at the beginning to load the Parent node into memory), it executes perfectly and updates the Child nodes only if the Parent's referenced Actions are not already present.

The code looks fine to me so I suspect there is something going on inside Rules that prevents the code from being executed properly. Any help would be much appreciated.