I would call your fabricator.

Most likely someone stood on the counter.

I was thinking what may_flowers said.

Oh my, I thought quartz was super durable? I used to stand on my granite counters to clean the cabinet tops, I never considered that would happen with such a tough stone, isn't quartz suppose to be just as tough?

Just a thought and could be wrong but I'm just wondering if there could be an expansion/contraction issue with the heat given off by the dishwasher. The counter won't be affected by it but because the seam is right there, the counter over the DW will be expanding and contracting but the piece next to it won't be because of the seam and might be producing or causing stress.

I stand on my quartz, and so has my contractor. Don't know why it should be more fragile than granite.

You should never have a seam at the dishwasher. And you shouldn't stand on your counters either. Granite or quartz. That's what articulating ladders are for.

In addition to never putting a seam at the dishwasher, are there other caveats regarding seams on quartz (or other) countertops? I realize each installation may be unique, but are there general "rules of thumb" we should be aware of when the templating and installation take place? TIA

I stand on my quartz on occasion. It's sitting directly on 5/8" plywood, if that makes any difference. Carrying in a ladder tall enough for me and futzing with it poses a degree of risk for me that I guess I'd prefer impose on the quartz instead.

If there is a crack what kind of filler can you put on it?

I'd contact your installer. I'm about to have Quartz installed and my fabricator has a 10 year guarantee.

I would think the crack was there all along but it has become visible now that the countertop has been in use. Back to the fabricator.

"I would think the crack was there all along but it has become visible now that the countertop has been in use."

I tend to agree. Regular use has contaminated the crack making it visible now. I'd hit it with some beauty shop peroxide, then tint some superglue with a color matched nail polish, and scrape it flush with a razor blade. Might disappear, might not, but it's worth a shot.

I just installed quartz and was advised I needed a special bracket due to the dishwasher vibrations I was told my counter would get a crack from the vibration if the dishwasher was not secured correctly.

That is nonsense. Modern dishwashers are attached to the cabinet sides, not the countertop. This is the bracket you don't need.

Hopefully this resolved. You should contact fabricator who will contact qtz manufacturer. All quartz has warranties against defects and if installed incorrectly fabricator should stand behind work regardless of warranty.
Yes excessive steam heat from dishwasher can cause cracks (thermal shock). Cold rapidly to heat.
You should never stand on ctops - never. They are too costly to risk.
but having a full 5/8 underpayment like one poster said helps with support.

something hot was put on top

Yep, likely from heat, which is ridiculous. I have a very similar crack on my quartz countertops that were installed April 2020. I recently used my crockpot on the counter and the crack appeared off of the seam just like yours looks. I am so upset. I was told by the installer that I could put hot things on the counter, but a quick google search after this happened shows that is not true. The installer is telling me it must have gone from super cold to super hot very quickly, this is not the case at all. They will not help us at all and it looks like i will need to get the corner piece replaced.