I've used the techniques in this tutorial to add a couple new managed properties to my custom item template which is selected by a search result type and has been working for several years. The correct template is selected but the mapped properties I added are not returned.
If I look at the refiners available to the search web part I see the added properties from the template. But they do not appear in the item information returned by the search.
The mapped properties are returned as expected when the search web part sets the template explicitly with the "Use a single template to display items" option.