We use declarative pipelines with yamlMergeStrategy merge() and a yaml entry. Some of the YAMLs have an initContainer section that is preserved when we do not set the merge strategy but get dropped from the final yaml when we do set the strategy to merge().

https://kubernetes.io/docs/concepts/workloads/pods/init-containers/

For reference the merge() option was added to the declarative pipeline in https://github.com/jenkinsci/kubernetes-plugin/pull/638