The bug

When lava spread fire, it can sometimes calculate the blockstate for the fire from the block beneath the fire. This bug can be replicated as so:

Doing so can sometimes create results like such: (Increasing the random tick speed will make it happen faster)

Code analysis

Specifically, this bug occurs in the random tick method of the lava fluid, it will calculate the fire state for the block below where the fire will be placed, causing the above result.

Code analysis by PR0CESS can be found in this comment.