Should all internal node keys in B+ tree also be in the leaves?
The internal keys come from values also stored in leaves, but if you allow deletions, the value could be deleted from the leaf after it is created and used in the internal node.  Deleting the value from a leaf won't change any internal nodes, unless the leaf becomes underfilled.  With the insertion you list, it would have the property you are thinking of if there were no deletions.

On the other hand, it is possible that the author wasn't thinking of either the property you name, nor the possibility of deletions, when making the question, and just used 10, 20, 30 as easy separators.  For sure, I have written questions with mistakes before, and usually, even with a mistake in the question, it can usually be answered properly to see if the student understands the material (insertion in this case).  If a student then asked me what your post asks?  I would be pleased with their understanding of the material to notice this property.