Water wisteria, or Hygrophila difformis, can have pinnate, lacy leaves or flatter, broader leaves. The leaf shape changes depending on its surroundings (light, temperature, nutrients, etc). All of my tanks are low light and the leaves have changed from the thin lacy type to the flat broad type. I actually like the variations in leaf form.
Usually the finder leaves are when it is grown Emersed so it grows much faster for more profit. With some plants it can be annoying because they melt so much if you do, but some plants will already melt.
So you're plant is adjusting to its submersed form, which I find really attractive