Confirmed. Yes, those should be showing dropzones. Definitely something wrong there, sorry about that.
To confirm, once you make an element a Slot, its children in the component builder will be ignored when the component is used in the page builder. This is so you can build with some kind of sample content in the Component builder to take up that space.
A good way to get the best of both worlds is something like this:
- Create a Component Set for low level components like Buttons/Headings etc. Let’s say we have one called “Button”
- Create another Component set for compositions that are bigger arrangements of elements. Let’s say we have a Section exported with the name “Top Level”
- Drag in your Button component (from the other set) anywhere inside Top Level.
- With that Component instance selected, toggle “Pass Through” from the element manager.
When you use Top Level in a page, you’ll get something like this screenshot:

The Button will be like a virtual child that can be individually inspected and edited. It can’t be deleted or duplicated because it is hardwired into “Top Level”.
Once that’s in place, you can add a Div or Column as a Slot to expose the a dropzone to use as well.