Spreading fire from lava is a known game mechanic.If there's a block like trap door immediately next to lava then fire will directly spread from lava.to the next block after trap door.
But in this case fire won't spread in North and South directions like this as shown in the picture.
To recreate this put everything as shown in the image and use constantly ticking observers below lava, which will spread fire faster to easily visualize the issue.