Dieffenbachia leaves are large and heavy and will bend down easily, sometimes causing the midrib to crease. That looks like what has happened with yours. They may also have gotten bent just a bit too far during repotting. The leaves are easily damaged.
Make sure the drain holes are not clogged. It's important for excess water to be able to drain out. The top inch of soil should dry out within 10 days. If not, give it less water next time you water.
As your plant continues to grow it will become top-heavy. It will then need ever taller stakes or it will have to be pruned back.
