ECS stack during pipeline deployment, didn't overwrite/delete the older tasks. Just added the new tasks.

We usually have two tasks running in our ECS. During a recent pipeline deployment within our ECS stack (to make a change in the ECS code), we encountered a scenario where two older tasks were not updated as expected. Instead, two new tasks were created alongside the existing ones. Subsequently, when reprocessing messages from the Dead Letter Queue (DLQ), some of these messages were picked up by the older tasks, leading to recurring failures.

We had to manually kill the two older tasks in order to resolve this issue.

I am seeking insights into the potential reasons behind this behavior. In a typical scenario, I would anticipate the older tasks to be updated or replaced rather than coexisting with new instances. Could this anomaly be attributed to a scaling issue, or are there other factors that may have influenced this behavior during the deployment process?

Additional Info -

• The older tasks which we had to kill were created on 6th December, and the changes in code were very minor which couldn't have caused the issue.
• This is our ECS which contains 2 tasks usually.