My baby only poops after wearing her new diaper and it became a habit for her. Any idea on why she does that?

I am surprise your baby is only pooping when wearing diaper! This means she is really smart to be acustomised to this "clean habit". I facing the same situation when I potty trained my daugther when she was 2 years old. She only pooped in diaper but not potty. It took a while for her to get used to poop in potty/toilet. Your baby is still young, so don't have to worry and it could be just a phase. After some times she might not doing it anymore. Although it sounds like a bit wasteful to poop in new and clean diaper, maybe you can try to put a nappy liner on top of the clean diaper and just remove the liner with poop and reuse the unstained diaper.