That should have been mitered to follow the rise rather than notched. And you should have s mitered return as well.

Molding should be cut on an angle to rise up over the railing, not be notched and keep that same height.

The molding is then cut short of the end of the wall by the thickness of the molding so that you can create an angled return to the wall.

You do have a miter saw, right?

The Pro. Is showing you the correct way of doing it in the photos