Need more photos, different angles, and from farther back.
But it looks like it may be mill marks in the veneer the way it book matches in a chevron pattern at the veneer joint.
If so, more sanding before stain/finish was applied might have eliminated?

Go to the shop and ask to see this on an unfinished piece of veneered ply. I think it's a defect. Check for full sheets without this. And tell them the defect is not acceptable for your job.

Thank you. I have delayed the countertop install, until this can be resolved.

Here are a few more images from different angles.

Medullary rays.

Sophie that was my first thought too but then I went back and looked at photos of my cherry cabinets from my condo. Those look too even to be medullary rays.

Looks like you've got some lovely curly cherry--or it would be lovely if you could see the wood through the unfortunate dark stain.

It's curly cherry figure, so it is completely natural; desirable, even, but the pigmented stain muddied and ruined the effect.

Thank you for the pictures, the markings on our cabinets look too angular and even to be medullary rays or curly cherry, to me. But maybe that is possible.

Could they be sanded and re-stained? Or do I need to push for a full replacement?

When I asked what it was during install, I was told "marbling" but have yet to find anything out there similar. This was not on the sample board we approved prior.

it is not just one panel it is multi places around the kitchen and the doors have not even been installed yet. If the custom cabinet maker did not sand it properly or stained it in a way to ruin the wood grain, why would they not have to replace it? It looks like it has car tread all over it.

Also there are issues with other things.

We chose them based on being a top custom cabinet maker rated on Houzz and receiving the "Best of Houzz" ranking multiple years in a row.

We were sold on the claim of top quality material, install and finishes. But these have been the results.

Yeah, they missed a little stain in the above pictures, but the waves aren't saw marks and are completely natural. You'll have to sand through the veneer to get rid of them. Learn to love them, please.

The marks are not coming out, and the stain is into the grain too deep. You need new components hopefully all the cherry ply with the curly veneers has been used up and it will be a plain figure.