The bug

Soul fire extinguishes when the block beneath it is set to any block that isn’t soul fire conducive. This is inconsistent with normal fire, which updates to soul fire when the block beneath it is set to soul sand/soul soil. 

How to reproduce

1. Place soul sand/soul soil close to a block of netherrack and light a fire atop both.
2. Replace the netherrack block with soul soil using the setblock or fill command
3. Replace the soul soil/soul sand with netherrack using the setblock of fill command
   →  Notice how a fire-to-soul fire block update occurs but a soul fire-to-fire does not