The correct answer is The action was changed from Create sub tasks to Create issues using the sub task issue type.

When the action was swapped in this way the rule may still create an issue with the sub-task issue type but it does not behave the same as the dedicated sub-task action. The dedicated sub-task action attaches each created item to the trigger issue as a child by default. If you use a generic create issues action you must explicitly set the parent or use a branch to associate the new issues with the original issue. This mismatch commonly results in only one properly parented sub-task or unexpected single creations when the original rule expected multiple sub-tasks.

The automation actor does not have the Create Issues permission is incorrect because lack of create permission causes the rule to fail or log permission errors and it would not silently create a single sub-task. Permission problems are visible in the audit log and do not explain why one item is created.

The rule is configured to run only once per issue so it only executes its post actions a single time is incorrect because the symptom describes a single execution that creates one sub-task instead of multiple creations within that execution. A rule limited to run once per issue would prevent repeated triggers but would not change how many child items a single execution creates.

The branch that iterates to create multiple sub tasks is missing from the rule is incorrect in this specific case because the rule did execute and produced a sub-task. If an iteration branch were missing you would expect the design to have no iteration at all and the rule would not attempt multiple creations. The observed behavior fits a changed action type more closely than a missing branch.

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