Hi @jkdeepnet! Thanks for reaching out. What you are describing is a known issue where if a node contains multiple parents, we (currently intentionally) pick the path without run data to start, and n8n just fails. Work is scheduled to begin on this soon, though the amount of work is high.
That said, the merge node is working based on how you’ve configured it. You may want to configure it so that it waits for both inputs to arrive however.
Here’s a modified version of your workflow where you can see one way to handle this.