In the Modern UX when you go to a cost plan properties in the fly-out and change the period start or finish, this actually updates the cost plan dates for cost plan detail.
This is working as designed and is similar to editing the dates on the cost plan properties in classic view.
Component : CA PPM SAAS FINANCIAL MANAGEMENT
Working as designed.
You can also change the default values that appear for the project START PERIOD
and FINISH PERIOD
fields. If you change the start or finish periods to a smaller date range, the application removes detailed data from the cost plan.