This issue is related to the Modern UX's "Financials" tab where it is possible to work with Benefit Plans.
Steps to Reproduce:
Expected Results: The Planned Benefit gets updated to 2000 since a row got deleted. This is how Cost Plans work in the Modern UX.
Actual Results: The Planned Benefit remains at 4000 even though the Benefit Details has only one row with 2000 as the Planned Benefit.
DE62211
Release: 15.9.3
Component: Financials
Fixed in Release 16.0.0