Yes, mangosteen is known for fixing stommach issues. I think there still is an autoimmun component in POIS, but its not related to sperm, its' related to hormones and the intestines in this way:
1. When the intestines leak undigested substanced and proteins, the immune system react to this and destroys the bodies own cells, including the hormonproducing- and hormone wast-processing organ (the liver).
2. On orgasm, your ability to break down the inflammatory hormonal by-products are compromised so they stays in the blood system and have more bad effects on the organs, skin, brain etcetera in a vicious circle.
So there's two autoimmunity processes at the same time; the first is the most easy to fix - the gut problems. reduce stress and get on a SIBO-protocol, totally stop eating gluten and any grains and do anything that is good for the stommach. Mangosteen juice is maybe a miracle product, who knows. Combine this with strategies to reduce inflammation (POIS-protocol) and hormonal improvement, and restrain from orgasm for a while.