I would expect mobs to burn orange regardless of whether they are positioned in soul fire or normal fire. Soul Soil is what burns blue. Netherrack and other substances burn orange. Soul Soil is used to create soul fire torches. I agree with jonathan schaudt.
A ghast could fire an orange-burning ball, setting soul soil ablaze blue. Anything the blue fire catches should burn orange unless it is crafted from soul soil or is a "soul mob".
I do not understand why people perceive a bug. I do not understand how it appears that there is something wrong with the game. Either way, I couldn't really say that there is something wrong with the game. The dumb game can have its own rules for fire, realistic or not.