This has been bugged since May 2018, over 2 years. Despite the clear description in the catalyst, only deaths from actual burn damage currently count, and probably will forever. Some people say the leviathan castellum area was a good spot, I found full health acolytes during a heroic adventure that I could reliably plant a 5th shot on their body and watch them die from the burn. It's important you make the 5th shot a body shot, otherwise the explosion will kill them and not the burn. Good luck and don't forget to preorder Beyond Light.