The content model for template styles sheets isn't correct. Just created w:no:Mal:Example-template-styles.css, and given the .css suffix it should be given the sanitized-css content model. On inspection it has the wikitext content model.
TemplateStyles is just enabled (T193786) and I guess it is just some additional configuration that is needed.
I have requested a manual change of content model, so it might have changed when someone checks this report.