How to view cards in the stack below the top card in MTG Arena?
There are two things two answer that question:


You can simply examine the cards on the stack by right-clicking on them. Then it will open in the large preview (usually on the left on the screen).
It is nevertheless a design flaw, as the way of doing this is not very intuitive (in my point of view) and also not "real like", as it would be if the stack was unfolding while hovering over it.