Why Is the Test Step Output Missing When an IF Node Precedes a MERGE Node in n8n?

Below is an explanation of what’s happening in your workflow:

What’s Occurring

When an IF node precedes a MERGE node in workflows built with the legacy execution order, the MERGE node can trigger execution of both output branches of the IF node—even if only one branch (typically the “true” branch) is meant to produce results. In your case, even though the flow runs as expected when executed normally, the “test step” function doesn’t display output because the MERGE node’s behavior causes an extra (or unexpected) execution path which isn’t handled by the test step in the same way.

Why This Happens

This behavior is documented for workflows using the legacy execution order (workflows built before n8n version 1.0). Specifically, when a MERGE node is added after an IF node, one branch of the IF node (the one that normally would be ignored) can be executed because the MERGE node triggers it once it receives input from the other branch.

How to Resolve

1. Review Workflow Execution Order:
   Check your workflow settings to see whether you’re still using the legacy execution order. If possible, switch to the new execution order (introduced in v1.0) where this behavior has been removed.

2. Refactor Your Workflow:
   If switching isn’t an option, you might consider refactoring your workflow to avoid placing a MERGE node directly after an IF node or designing the branches such that unexpected execution doesn’t impact the output of your test step.

3. Testing Note:
   Remember that the “Test Step” functionality might behave differently than a full execution if it doesn’t process all branches as expected. Running the full workflow might produce the intended results even if the test step doesn’t show output.

Summary

Your issue is a known quirk from legacy behavior in the older execution order, where placing an IF node before a MERGE node causes both branches of the IF to execute. Although the flow works during normal operation, the test step can appear empty because it doesn’t capture the side effects of the extra execution path. Consider updating your workflow to the new execution order to avoid this behavior.