Hiroshima had the largest church in Japan. A great amount were killed when the bomb came. But also some had dreams to leave before it was dropped and they survived. There were different christian types. There was a christian monk who survived -right after devotionals he was having breakfast and then the bomb came, Total destruction around the area around him except the building he was in at the time. He survived no cancer for 30 plus years going to churches around the world as a witness.
Yes, I’ve heard of the Hiroshima miracle survivors too. There was a group of German Catholic monks, who, despite the Japanese regime’s hostility to Christianity, were tolerated and allowed to continue their work because of their German citizenship - Nazi Germany being an ally (the Lord works in exceedingly mysterious ways sometimes). They were basically at ground zero, yet survived with no long term effects. Maybe the same monks you were talking about.