First of all, are you sure those people you met were INFJ / ISFJ ?
I had someone who I thought his type is ISFJ, but turned out it was INTP . And, another who managed to fool everyone acting like INFJ, but he was ISTP. The problem was the accuracy of the test.
What makes the test accurate in it's result, if the person who answer it is a human being? What if that person choose different answers for some reason?
I still don't know much about MBTI theory, I read books, but I still find it not clear yet. So, maybe I was wrong about their type too.
Instead I met arrogance, abusive behavior, the occasional bragging about them being like god, when it comes to their abilities or sheer ignorance.
I met an INFP who was prideful, and I was surprised. It's just like how true colors said that blue are emotional and care about Feelings and all these "lies" as fact. Like they are the "harmless, clean and fluffy" being in meaning.
That INFP was annoying when he was giving me the passive-aggressive behaviors. And, he had stupid reason that is irrelevant to this topic.
As for ISFJ, I know one now. She mistaken herself to INFJ , but then she found out that she is ISFJ. She remained the same before and after learning her type.
There many things missing in MBTI theory. For example, someone in this forum who was speaking as he/she is ISFP, but then found out that his/her type is ESFP.
Take an example the cognitive functions, there is 8 functions, and yet these functions are still not clear. ISFP main functions are Fi Se Ni Te according to the MBTI theory
And, ESFP are Se Fi Te Ni. I also remember somewhere that there was an INTP who thought that he was INTJ and there is a big different between Dom-Ne, Aux-Ti and Dom-Ni, Aux-Te.
Another example, maybe my type as ESFP is not right too. Maybe it is, but I'm 5w6 and Green in true color. How can someone prove that?
MBTI is useful as a way to "explain" some behaviors, but I think all these description for each type is still missing a lot of info. Not to mention that a person development depends on the person environment and traditions.
Most people miss this fact. Either that person is INFJ or INFP or "any type you want".
If I were you, I will take Generalist advice. Depending if that expert is "really" an expert.