You can get normal fire on soul soil

The bug

Fire on top of soul soil can be normal fire instead of blue fire when normal fire was burning on the side of a block and soul soil is placed below.

Something similar happens when fire spreads.

How to reproduce

1. Place a two block high log tower

2. Ignite the upper log block on the side

3. Place a soul soil block below the fire
   ❌ The fire is moved on top of the soul soil block, but does not convert to blue fire