Hello Megan, I am also in the same boat as you. I have a large number of plants In my house and three of my monsteras are showing the same symptoms. The mother plant is around 7 years old and the other two are just a year old. They all have some black spots on them and the new growth on them is not healthy .
Did you ever find out the root cause for your problem? Hope you did.
Thanks for sharing.
