looks similar to Jambos as well, but your photos of the leaves not too good. Also the flowers are old or different than Jambos.
I have never ate Polycephalum, but from wikipedia it is said Polycephalum is sour, Jambos is definitely not sour.
I think there are others in family with round fruits/white flowers. Wikipedia says Polycephalum is found in Indonesia, but I believe Jambos is widespread.
I think can know more once fruit has matured and you have tried it!
Also Jambos new leaves flush purple, not sure if Polycephalum does as well.
Interestingly Polycephalum is said to have edible (possibly delicious) flushes of new leaves. Jambos new leaves I have not tried but have never heard of people eating.