Today some of my perfect fruit trees losed their leaves (I knew that would happen), but when I planted the perfect fruit in the same place the last tree was, the sapling looked dead already, anyone know why that happens?
Not sure. When I plant saplings, some of them are just randomly dead even if they're not near another tree/rock! Did you plant it too close to another tree? Or, did you plant the rotten fruit?
The fruit was rotten which often happens when the tree withers. It still looks like the normal perfect fruit but will be named differently. You probably did not pay attention to the name when planting it.
Another option is that since you had that tree you added more stuff in the area. There is a limit to how many trees and bushes you can plant in every block.