I have not been able to alter an existing view since converting from 5.1. When I right click on the view, select Operations and Alter View in Editor, the existing code for the view is not displayed.
If this is for SQL Server within TDP, then yes, it's a known issue in the latest versions of TDP.
One work-around is to select the view definition from sys.sql_modules, copy the resulting results code into the Editor, and then execute the appropriate CREATE OR REPLACE VIEW with your changes. Snap below shows how I am able to pick up the definition of view V_Customer.
Thank you Gary for the work around.