Need help understanding Python recursion issue for 103. Binary Tree Zigzag Level Order Traversal

Question

While solving Leetcode problem '103. Binary Tree Zigzag Level Order Traversal', I ran into issue that I am unable to understand why.

Following is the serialize logic I am using -

    def serialize(self, root):       
            if not root:
                return 'n'
            
            left = self.serialize(root.left)
            right = self.serialize(root.right)
            
            s =  str(root.val)+','+left+','+right
            return s 

To deserialize, I am checking the first character to take action and function is called recursively for rest of elements.

def deserialize(self, data):
        data = data.split(',')  # serialized string is comma separated with 'n' for Null 
        
        def buildTree(data):
            if not data:
                return None           
            
            if data[0] == 'n':
                data = data[1:]  # 0 index element is used. Use rest next time onwards
                return None
            
            root = TreeNode(int(data[0]))            
            data = data[1:] # 0 index element is used. Use rest next time onwards
            root.left = buildTree(data)
            root.right = buildTree(data)
            
            return root
        
        return buildTree(data)

However, this is not working. But following code with same logic is working. Just the change is I am popping the first element and not reassigning.

Difference in Above : data = data[1:] and Below code : data.pop(0)

def deserialize(self, data):
        
            data = data.split(',')

            def buildTree(data):
                if not data:
                    return None

                c = data.pop(0)
                if c == 'n':
                    return None

                root = TreeNode(int(c))

                root.left = buildTree(data)
                root.right = buildTree(data)

                return root
            return buildTree(data)

Can you help me understand what might be the issue here? As the list is mutable, it should work. Thanks