To put simply: Whenever you shoot an arrow onto a fire source (eg, campfire, lava, etc.), the arrow visually appears as if it's on fire before hitting the fire source itself. I don't know whether it has to be specifically a crossbow+soul campfire as shown in video, but that's just how it is for me

Link to video (can't attach because of size limit): https://youtu.be/EKPhpm1rafQ

I found this out accidentally