Hmmm ... I'm having a plant issue as well, but it is a little different. I will try to keep it brief, here goes ... I have a room saved on my gallery filled with plants that are mostly Perfect (2 are Magnificent quality). I put the room on 2 different household lots and they all devolved to Nice quality. I am currently building on another lot and have added this room. I am hopeful, but will not be surprised if these plants devolve as well. It's just really frustrating as my Sim is top level with the Juice Fizzing and top quality ingredients are a must for Masterpiece work.