If you “work in tech or web development” and you ‘“look at that error message and think, “So what? Her entry is clearly invalid according to the rules of the login form.”’, then YOU are part of the problem.
Some marketing team or manager or designer “designed” the form and told a programmer what to accept — a last name in this field. If the programmer wasn’t smart enough or competent enough to realize that not all last names match the same pattern, that programmer needs education.
I (a programmer) see this problem too often. The other day, indeed.com asked me of my email domain was “valid” — every time I set up an email notification. It’s bad enough that it asked, but it didn’t save the answer.
Forms insist that credit card numbers mustn’t include spaces, SSNs can’t include dashes, telephone numbers “must” be formatted in a particular way.
You’re a programmer. Your job is to check for bad data and collect good data for the back end database. But if your “good data” criteria are too narrow, that’s not good programming; it’s poor systems thinking.