Looks like matching stone only one had UV exposure and the other didn't.

That is awful. I would send it back. I don't know what the fabricator was thinking but they must have been drunk.

How could they not have expected loud and justified complaints after installing that?

Yikes, that is a do over. Are you sure this is quartzite? Have you tested with an acid for etching and A shard of glass for scratching? I ask because the stone does not appear to have the crystalline structure I associate with quartzite, though no geologist am I. Quartzite is expensive and I would ensure you got what you paid for, Check out Karin_mt's threads on how to distinguish quartzite from marble and granite.

So sorry for you, what a headache :(. I agree with everyone above. They need to install a new countertop with matching slabs.

I would want them to do it again. Very obviously doesn’t match!

They should replace at no charge.