The arrowhead looks just fine. The yellowing leaf is an old one the plant no longer needs. Remove it and continue doing what you’ve been doing. The Dieffenbachia is harder to diagnose.
How do you decide when to water?
Does the pot have a drain hole?
Does water come out the bottom when you water it?
How close to a light source is it?
Have you fed it? If you did, what, how much and how frequently did you use?