Hello DineshV-1571
Welcome to the Microsoft Q&A forum.
Using CICD it only merges the changes, it doesn't delete/add any objects.
In your case, every pipeline got deleted that didn't exist on DEV - this is because of the above reason I mentioned (CICD only merge the changes)
The pre and post deployment scripts stop triggers before deployment and restart them afterward.
and your understanding is wrong with respect to -deleteDeployment $false
The -deleteDeployment flag is used to specify the deletion of the ADF deployment entry from the deployment history in ARM.
https://learn.microsoft.com/en-us/azure/data-factory/continuous-integration-delivery-sample-script
If you are looking to cherry pick specific pipelines using CICD, you need to consider isolating the artifacts in a separate branch and use the validate and deploy operation to deploy the artifacts from that branch.
https://learn.microsoft.com/en-us/azure/synapse-analytics/cicd/source-control#unsupported-features
I hope this helps.