Chris: Fabricators are promised by manufacturers to get 120" out of standard quartz slabs, but can often sneak another inch or so out. Each slab is different. If they promised no seams, that's what they should have delivered; they know how big the slabs and your cabinets are. Fortunately your seam is large enough to saw apart with a Japanese pull saw: It is also good that the sides of the seamed pieces appear parallel. This one wouldn't be a difficult do-over. They tried to do a wider particulate-matched seam instead of a tight color-matched one. That works for me for granite that doesn't fit very well, but this should pull together nicely. It appears they did a small finished return end at your wall, like you would do for a slide-in range opening. Somebody didn't check the print or template. It won't be nearly as conspicuous after it's caulked. Tell them you'll live with it if they redo the seam; it makes you appear reasonable.

Thanks for the responses. I guess I was comparing it to solid surfaces like Corian which seams are usually not as noticeable. I didn't realize quarts would produce a seam like this. I can live with it.

Diagonal seams are not acceptable. They are the weakest, require much more support, and have the most chance of a color and pattern mismatch because they are the largest possible joinery seam. They have the most chance of shifting as cabinets settle and move, giving you a large area of lippage vs a small one. They also waste the most material, which costs you more with the extra slabs needed, and in general, just look ugly with any stone with directionality. That diagonal is not a good look. At all. . A french miter is always the preferred seam for a corner. Quartz is not natural stone. It has zero predictability in color pattern production. You need to have that expectation on the front end. You will not get a pattern match like bookmatched natural stone. Ever. It's not printed from a pattern like tile. The individual solids colors are mixing as swirl paintings that are different every single time. No manufacturer or fabricator anywhere warrants a seam to be a perfect match, because the product has no predictability. Each slab is individually patterned and colored and is unique, and can not ever be a perfect match. That is a as well done a seam as any Quartz manufacturer would promise. It an acceptable match. It does not fall outside any industry tolerances.

These days, one can never assume that "professionals" know the proper way to do many things. One always needs to ask first, and yes, we all have tales due to our lack of experience. Sometimes people will ask me why I'm so cynical. I always say I was not born that way but became that way due to bitter experience!