I present very flat, nearly at all times. And I think many NTs find it very rude if you do not engage emotionally/visually with them.
I have no real idea why, this is my normal expression. But I guess when I do try to use facial expressions and body language.. I tend to do it at the wrong times (I have been accused of smiling at some very wrong times, laughing during others.. 'having a bad mood' when things are suppose to be exciting, etc)
I can tell when customers, I work in retail, are trying to read me specifically because they tend to get very snippy and very prompt with me (like I ask for their photo id, and they toss it on the counter instead of putting it in my hand). I'm not doing anything wrong, I am enacting exactly what my job details, my job does not require me to smile or make conversation that I don't really understand with others. They came up for a service, I'm doing it for them in a timely manner. That should be enough, I am friendly in my own ways.
I still hear remarks that I need to smile to offer better customer service, I should stop looking so unapproachable and angry, etc.. My flatness carries over onto my voice, especially on the phone (the phone is so unengaging.. I greatly dislike phones). Many people will not converse with me over the phone because they can't stand it (the flatness) and the lack of consistency in my speaking pattern. It isn't intentional, sometimes my brain just goes too fast for my mouth and I need to restart what my brain was thinking, unless the question is something I've scripted for before.
I can't say I still understand why flat affect equals everything that has been said/assumed of me, I'm naturally content and generally neutral. But I just go on with how I am because I'm not interested in changing that aspect. If people don't want to understand, or if they want to be critical over flat affect, then they can go ahead.