You’re describing, pretty perfectly, how confirmation bias works, and how such mythology perpetuates. You are not, however, describing anything remotely related to empiric evidence. All of your evidence is positive; i.e., anecdotal by nature and consisting purely of confirmation. Real proof must include negative proof; i.e., other explanations must be eliminated. A clumsy description, but perhaps the point is made nonetheless.
Example: there’s a whole bunch of confirmation evidence that the sun goes around the earth. That hypothesis, however, does not stand up to negative scrutiny. Likewise, there’s a whole bunch of confirmation evidence that does not disprove the hypothesis that underlies dog breed prejudice. But lack of disproof does not equal proof.
Think of a criminal case, like you see on TV, where all the evidence perfectly lines up with a theory of the crime that convicts someone of the crime. Only to find out that he was not in fact guilty. That’s how confirmation bias works. I have a theory. This bit of evidence supports it/does not disprove it. So does that bit. You can have as much confirmation evidence as you want–as in with breed prejudice–but all you need is one piece of negative evidence, that disproves it, and it’s disproven. A million points of confirmation proof is still not proof: any hypothesis must be tested against negative proof. That’s never been done with dog breed prejudice, so people have–for millenia, as you point out–accumulated confirmation evidence that seems to support the prejudice.
When, in fact, it’s not the case.