Wish I could help but I don't see a feathering tutorial in my iteration. (I'm on Android.)
Out of curiosity, does feathering work outside the tutorial? (Make a selection > Selection option icon (4th icon from top left) > Feather).
Good idea that I hadn't thought to check. Either I'm doing this wrong, or feathering doesn't work.
Author of The Natural Bible
205 Howland Ave.