Lava spreads blue fire at invalid locations

The bug

If lava is at the same height as a flamable block (e.g. a sign) on top of soul soil, blue fire is placed above. Even if the location is not valid for fire.

How to reproduce

1. Place lava
2. Surrounding the lava place at the same height signs with soul soil below
3. Increase the randomTickSpeed gamerule if necessary
   Blue fire is created above the signs even though this location is not valid for fire