When you create a Relationship Custom Field and add a task to another List, the task loses its custom relationship field.
For example, in Sprints, we have a Backlog with all tasks grouped by Product (Product comes from the Product Roadmap list) using a Relationship List. When we add a task from the Backlog to the Sprint Folder, the task no longer stays connected to its Product.
We need Relationship Custom Field values be available and stay the same when a task is added to another List as other custom fields do.