ContentService Dependency Injection Not Working

I've created a Custom Servlet Plugin which has been working fine for a year now.

While it does not say this in the Custom Servlet Plugin documentation, I've noticed in the Custom Smart Service Plugin page that the ServiceLocator.get* methods have now been deprecated and that dependency injection should be used via constructors.

This works fine in my custom servlet plugin for the ProcessDesignService, but this is failing for the ContentService where I get an exception thrown of NoSuchBeanDefinitionException.

In the Custom Smart Service Plugin it states that:

Any of the *Service interfaces in the public API (such as, ProcessAnalyticsService, or ContentService) can be obtained by passing them as parameters to the constructor.

Is this a case of the ContentService just not able to work with Custom Servlet Plugins, but fine for Custom Smart Service Plugins? If this is the case, then what am I suppose to do? Leave the ServiceLocator.getContentService deprecated method until this issue is resolved? Or is there something else that I am missing?

I am using Appian 18.1 by the way.

Any assistance is appreciated!