I get a similar error with a different root cause compared to: Clone Graph LeetCode 133

Consider the below implementation. If I use a Node-type key for processed_node_map, the algorithm passes. If I use node.val as the key, I receive the error:
Node with value 2 doesn't exist in the original graph.

Question: Can anyone explain why this error is occurring? TYIA

Constraints and notes:

1. Node.val is constrained to be unique across nodes in the input.
2. This error is seen in a test case with no input value of 2.

class Node:
    def __init__(self, val = 0, neighbors = None):
        self.val = val
        self.neighbors = neighbors if neighbors is not None else []

from typing import Optional

processed_node_map = {}

class Solution:
    # solution: https://leetcode.com/problems/clone-graph/solutions/5092202/blind-75-beats-88-06-46-75/
    def cloneGraph(self, node: Optional['Node']) -> Optional['Node']:
        return self.dfs(node) if node else None

    def dfs(self, node):
        if node.val in processed_node_map:
            return processed_node_map[node.val]
        
        clone = Node(val=node.val)
        processed_node_map[node.val] = clone

        calculated_neighbors = []
        for child in node.neighbors:
            calculated_neighbors.append(self.dfs(child))

        clone.neighbors = calculated_neighbors
        
        return clone