Hello Dhanavath,
What you experienced is the expected behavior: when you remove the tag or any other target condition for your defined automatic deployment is no longer valid, your existing device is removed from the deployment, see also documentation on Automatic Deployment.
If you want to have a specific deployment for your devices, for example they got the initial deployment and you want to test a new module before deploying it to all devices, you will need to create a new automatic deployment with a specific manifest and a specific condition what has a higher priority than your initial tag-based deployment.
Please accept the answer and feel free to upvote if your issue was solved. If you still have questions feel free to provide more details.