If you look at regular fire, it makes some black smoke particles that appear to come out of it, but soul fire doesn't, here is a image for comparison:

Steps to Reproduce:
1.Light a fire using flint and steel on netherrack and soul soil to make soul fire and regular fire
2.Pay attenction to the smoke particle that come out of it
3.That particle only appears on regular fire

Observed Results:

There is no particle produced on soul fire making it look a bit blocky (in a bad way)

Expected Results:
Soul fire should have that smoke particle coming out of it like regular fire