This issue is related to the Modern UX's "Financials" tab where it is possible to work with Benefit Plans.
The steps to reproduce this issue are as below:
Expected Result: The Planned Benefit gets updated to 2000 since a row got deleted. This is how Cost Plans work in the Modern UX
Actual Result: The Planned Benefit remains at 4000 even though the Benefit Details has only one row with 2000 as the Planned Benefit
This is caused due to a defect with ID DE62211.
The defect is fixed in 16.0.0.