It's hard to tell which leaves are which from your description, but you're right, the leaves from below the graft can come up and look different. If the plant is younger or gets set back the leaves look different, too, since the leaves from the mature or healthy plant are often finer. It could be that the new growth is larger from being set back by your removal of the original growth, but whether that growth was from the nurse root or the tree peony is hard to say without seeing it. You could post photos of the different leaves to help make it easier. The link is an example of a yellow tree peony with a photo of its leaves, but it may not match exactly since each peony has subtle differences in leaves.

tagzone5

Here is a link that might be useful: Alice Harding peony