Just finished watching Netflix's Dark. I couldn't find answers for this, how is it that?
Hannah in 2019 could not spot that Mikkel is exactly the same as how her husband Michael looked like in 1986?
If we assume that they were living in the same village all these years, Hannah would probably know Mikkel since his birth, when he would definitely wouldn't look like Michael. Then he would grow up, gradually changing to be more and more alike the kid she met at 1986, but for her he would always be "Mikkel". She would never have thought that "this kid looks familiar" because she already knows him. Even if she would think that he looks like her husband when he was young, then she would think that it is just a funny coincidence.
or... plot twist! She saw the resemblance and she thinks that he is the son of her husband and that's why she wants Ulrich so much, to avenge Katharina for sleeping with her husband.
OR! Bigger plot twist
Mikkel is Michael's son.