Crimes of Grindelwald made me think; what if he was saved at the last second with the Bubble-Head Charm, conjured an air bubble around him and etc. and is/was still alive, and hid him for all those years without anyone knowing? A witch or wizard could've possibly, somehow preventing the family tree box from showing that he's still alive, unlinked him from it, so it can't show him growing up, or found a way to create a realistic illusion that only showed him as a baby when in reality he was all grown up to hide him from his father and his family's legacy. Unless Leta or anyone else saw him die and/or his dead body, this could actually be possible.