As-is:

There is always created an estimate line with the 'Category' on the task (even if 'Effort (hours) is 0.00), and the 'Category' on the task is mandatory.

When you make changes on the task on 'Scheduled planning' and you existing hour estimate lines on 'Estimated costs and revenue' with different categories in addition to the automatically created line, then the hour estimate lines are deleted and replaced with a new single line with the main category on the task.

To-be:

We should have a link between the automatically created hour estimate line and the task. If there are other estimate lines with different category from the task when changing any on the task, then this line should be updated with the changes and the other hour estimate lines should remain as before.