I found this closed topic asking the same thing. It was custom collection under custom collection. Which official response is to workaround for the time being.
I tried to have list collection nest under custom collection, it did work as shown below
As you can see, I need to group diary entries by date. With my limited knowledge, I can't think of any other workaround
However, rather then the preview auto update/populate on change, I now have to refresh the editor page to see changes.
Is this expected? Or have I made accidentally made something that's not planned and wasn't support to work?