Hm, that's right. It's defined as a path. I doubled clicked a new box and saw that it was converted to a path, that introduces a second issue cause the only way for me to actually select a box that is grouped with a bunch of other elements is either ungroup a bunch of nested groups one by one, or double click the box in order to select it. If doing that converts that element to a path then it makes selecting very tidious. The example you saw was just the box container seperated from a nested layout with heaps of stuff going on inside that container. Can't ctr+click either like some tutorials claim as that doesn't select the single layer but always something surrounding it as well. Makes it even worse if that whole container and its content is a component.
PS. I shared a new doc with a new box that wasn't a path to code specs and corner radius was spesified in the specs as expected.
Here's how it works.
Let's say a box is in a group with other elements.
If you click once - you select the group.
If you double-click - you select the box.
If while the box is selected, you double-click again, then it makes it a path.
If you don't edit anything while in the path phase, and you deselect it, it goes back to a rectangle and it's all good. If you edit anything while the path is selected, it stays a path.
Alternatively, you can Ctrl+Click on any element to select it directly, without the need for double-clicks and so on.