Not quite sure what "sincerity" means, even after looking it up on Google I still don't see how it suits the question the OP is asking, so I'm not sure how to answer.
I'll give my two cents anyway, and sorry if this is irrelevant.
I've been told by close NTs whom I trust that a lot of NTs can get a little creeped out by people who come across as way too nice. It might sound baffling to most Aspies, because you're all probably thinking "but what's so wrong about being nice?" There's nothing wrong with being nice as such, but it doesn't always earn respect. Some NTs even think that people who are too nice all the time are sissies.
I can see where they're coming from and I can see why it sometimes may creep them out.
I think social rules ideally are best learnt the hard way through experience, rather than taught logically, because there are no precise rules. Every social rule depends on the context of a situation, environment, what sort of person you're talking to, what sort of person you are, and so on and so forth.
Many Aspies fail socially or romantically because they are only going by how they've been told to act and then they tend to do one extreme or the other. But sadly it doesn't work like that in the NT world, as I know all too well.
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Female