Kubernetes service deploying in default namespace instead of defined namespace using Helm

The most likely issue here is that the Chart already specifies default as metadata.namespace which in Helm 2 is not overwritten by the --namespace parameter.

If this is the cause a solution would be to remove the namespace specified in the metadata.namespace or to make it a template parameter (aka release value).

Also see https://stackoverflow.com/a/51137448/1977182.