Pairi Daiza just announced on their Facebook page that sadly baby Sumatran orangutan Sungai, born October 30th, 2018, unexpectedly passed away last night.
The whole explanation of the cause of death given in the text, linked below (in Dutch, unfortunately) is a bit strange I think.
They speak about a congenital malformation of Sungai's jaw that led to him not being able to suckle anymore and him getting weaker. Medical examination and medical care could not save him as he was already too weak to recover. I find it a bit strange that this problem only leads to issues in an animal after nine months, then kills it seemingly very quickly.
Pairi Daiza NL Facebook - Death of Sungai
Possibly as he grew the jaw deformation got worse, at least that is what they imply, so the malnutrition worsened rather suddenly toward the end when it was too late to save him.