When I did studio and I add the project, next I chosen the icon for collection thumbnail all were good to time, when I removed from collection project which was icon. I thought thumbnail will change on default 'chest' but icon still was same. It's a bug?
It is not a bug, you see a collection thumbnail is an image. You want a thumbnail that is good for some collections but don’t want a project in the collection just for the thumbnail. The Snap! developers could add it so you can directly import thumbnails to make it easier. If this was a bug I’d say it is a good and helpful one.