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Is it in a pot? Did you move it or fertilize it when this started?
I haven't had this happen with pitangtuba, but I have with many other plants. It seems once the fruits stall they never resume normal growth, and will eventually color up as if they are fully ripe but still runty, and fall off. I have no idea what causes it... probably just generic "stress" of any sort. The next set of fruit should be fine.