Ok. I noticed something when I was re-watching the series a while ago.
Remember in episode 10, when both Madoka and Homura are about to become witches? Homura asks if Madoka has any grief seeds. Madoka says that she didn’t have any. But she later pulls out one after Homura is done talking about becoming a witch.
Looking closer at it. I noticed that it is Sayaka’s grief seed.