When a new code (a pull request or PR) is merged into a source code, it becomes part of that code and when the next release is published, that new code is also going to be part of the new release. The only way to avoid that is if the PR is merged into a separate branch that is not published when releasing the new version (like master being on v1.8 wanting to publish v1.9 next but merging the PR into future release branch v2.0) or if it is rolled back.