Assignments Start and Finish dates are changed when updating the ETC (Total) value in Assignments page in Modern UX
This occurs if in the 'View Options' page the default 'Sum of Periods' is selected.
Is this an expected behaviour?
Release : All
This is expected.
The 'Total' (SUM OF PERIODS) - will use the periods, spread the amount and will adjust the dates of the curve if necessary to extend into the periods from the view options.
Using 'Grand Total' - will spread the amount inside the dates of the existing curve date boundaries and won't change the dates of the curve based on the periods defined in View Options.