In this scenario, we have a route called "InboundAction" regiestered in the route collection, but the UrlHelper while generating a link doesn't find a match and returns null. In this case, we need to throw exception just like how Edit link generation behaves.
Currently we silently passthrough and the action links are not generated as expected. We should throw exception to let user know about the condition.
Example from Edit link generation convention
if (editLink == null)
throw Error.InvalidOperation(SRResources.GetByIdRouteMissingOrIncorrect, routeName);