People seem to be more understanding when it is a computer process that rejects something. "Oh, well, it's a computer."
But if it is a human they consider it unforgivable, not realizing the human is doing exactly the same: applying extremely simple rules to the situation.
It would not make sense to put a much higher paid human, with higher capability to make decisions, so that a few applications could save a few sheets of paper. It makes more sense to make the process as simple as possible so that it can be done by the lowest qualified person.
I can understand the motivation and the frustration of the OP but it is just a mistake and nobody is in the wrong.
I have learned to fill forms completely. Leave nothing blank. Write "N/A" or cross out if it does not apply. If the form is seven pages with a total of 75 boxes I will fill out every single one and check and double check each one. And there's a good chance a mistake still crept in so I will check again the next day.
I hate bureaucracy. I believe bureaucracy is a cancer that can be very damaging to a country and it should be kept to a minimum. Spain is awful in this regard. Just awful.
Also, most countries assign insufficient resources to immigration because immigrants can't vote and because many nationals are xenophobic and support giving those furriners a hard time. Immigration bureaucracy is hell pretty much everywhere.
But, having said that, if you are careful, fill and fulfill everything correctly, file it in time, and meet all requirements, generally things work out well.
Of course, you can always come across the asshole who takes pleasure in making others miserable. There's a lot of people like that in the world. And people who support them. It is a sad fact of life.