Browning of apple flesh is so quick. Bite an apple and you will notice that its flesh color turns brown before you made another bite.
Open a bangkok santol fruit, suck up one seed. You will also notice that peel turns brown before you finish sucking the first seed, unless you are mad enough to suck all the seeds at once.
This phenomenon is called Enzymatic Browning Reaction, a natural chemical process involving polyphenol oxidase or other enzymes that create melanins, resulting in a brown color. You have nothing to worry about it because it is completely safe. No one has been poisoned or hospitalized because of this occurrence.
I am not implying that it’s okay to leave your fruit open for a long period. Spoilage organisms are very notorious and will render food unsafe for consumption.