Idiosyncracy is like an odd behavior that a person does, or an odd habit that you personally might have. Everyone has their own idiosyncracies that sort of make them different and unique.
An anomaly is something that does not happen very often or under normal circumstances.
I am just comparing the two words in relation to people's behaviors.
So really my question is: At what point does something go from being what could be considered a normal behavior, to something that is considered abnormal (Idiosyncracy vs. anomaly) when it comes to Autism?
I'd say it's when it becomes uncomfortable to the other person. So if you're doing something, but they can get over it, then they're likely to chalk it up to you being you, but once it goes too far then it gets weird. But it's not just related to neurodivergence, I'd say it happens for all things, like if someone eats a weird thing or wears too out-there clothes then it moves out of the "yeah, whatever" to "oh, that was a bit weird".
Because of this, it also depends on what "normal" is to the other person.