Users are unable to Create new project template tasks after an initial project template is created from a project.
Release or Environment
This behavior is by design. To compare, when a Project Task is created for a Project, there are many scripts and actions that take place in order to sync, update and connect Project to Project task and Project task to any other child or parent Project tasks. Many of those actions take place for example when the planned_task Business Rule: 'Recalculate' is triggered.
In short, the quick response here is that the functionality to add Project Template tasks after the fact (of creating a template) has not been created for the system.
As this functionality is not yet implemented into the application, you can always create buzz or show our Product teams that this is something important to you and others, by searching our Community Idea Portal for a relevant, existing Idea and 'upvoting' it.
Otherwise, please create an Idea in our Idea Portal by clicking here. *You may need to log into the Community site.