I just tried to switch from our spa-extended to the new spa renderer. We should really mention that it is absolutely crucial to add the property:

class: info.magnolia.rendering.spa.renderer.SpaRenderableDefinition

In the documentation site her it is mentioned but as some kind of optional:
https://docs.magnolia-cms.com/product-docs/6.2/Templating/Template-definition.html#_common_template_properties

But we really should add it here:
https://docs.magnolia-cms.com/product-docs/6.2/Developing/SPA-development-and-Magnolia/SPA-hosting.html