When inserting programs into a Visual Studio COBOL solution, Visual Studio changes the dialect automatically without notifying user.

The reason why the dialect is changed in individual files which are imported to the project is because there is a syntax check.

The way to stop Visual Studio form changing the dialect on individual files is to go to menu:

Tools>Options>Projects and Solutions>COBOL Project Settings

Then untick the Set the DIALECT directive on files (you may need to remove the files from the project and then add them again).