My memory didn't quite serve me right, so it's not exactly as I described, and I can see, that as you say, providing the namespace to the template command does work.
The problem for me is, that setting the namespace in that way with "helm template" does not seem to add it to any manifests not specifically specifying the namespace to .Release.Namespace.
The rancher 2.6.8 chart does not set this for all manifests, but does with some, so when I set namespace though template, and deployed it all through the manifests folder, I got some objects in default namespace (because they had none specified), and some in the intended namespace; resulting in an installation that did not work.
As another reply to my comment suggested, this can of course be handled with post-processing of the result of "helm template", though, at the time, I was not certain the problem was limited to this namespace issue, so I didn't feel lucky enough to go down that route. :-)
When we generate the templates we use -n to override the namespace.
The command looks like this helm template CHART_NAME CHART_PATH -f CHART_INPUT_PATH/config.yaml --output-dir CHART_OUTPUT_PATH/manifests -n NAMESPACE --include-crds --render-subchart-notes --kube-version KUBE_VERSION