I think it's normal to assume whats going one someone's mind (prejudge) and make decisions based on it. Like, If I assume something about you and do something based on that assumption and you feel like I understand you, It's a different feeling, a happy feeling. But, it's a gamble, because what if the assumption is
incorrect, what if how I acted ruined the interaction?
So, I believe knowing a person requires a longer time then we want it to take and it will consist of mistakes and lucks that'll slowly form a decent representation of a person in our mind. For a tech nerd like me the best example for the process of getting to know a person is like training an AI model. Like, look at how DALL.E could generate images a few years back and how it generates images in the current state.
So, yeah! We don't know a person within a month or even an year or a lifetime because knowing is only predicting what the other mind would think and it's constantly changing and evolving. And when we think we know a person it's only a close enough representation of the actual person in our head compared to what they really are, similar to DALL.E making images out of probability and assumptions.