@jeffeb3 has a lot of good information.
I think I have a few good things going for me in this regard.
Walnut and maple are both pretty hard woods. Harder woods tend to shrink and swell less as it’s harder for moisture to get into them.
The walnut has also been 4s and sitting on my shop floor for 6 months at this point. Including through the winter. It’s pretty dry at this point.
The maple is only a few months old. I cut to length, 2s, and ripped it a week before doing the first glue-up. This would have given the wood a chance to shrink/swell as faces that used to be inside the board are now outside. When I went to re-check square after that week, the boards were still straight. This leads me to believe the boards were pretty dry when I started.
The only wood in the board I question is the purple heart. It was a last-minute idea and was given the least amount of time to shift.
The board will also have cutting board oil soaked into it. It should help to keep moisture out of the board. Being thicker, it shouldn’t cause much swelling on it’s own.
I’ve had a few cases where boards moved on me. It’s usually rough cut lumber that I’m jointing and planing down to my useable boards. I was taught that anytime you’re surfacing all 4 sides, that after each process, you should wait 24 hours. Then double check the board before moving on. This is because each time you open up a new face on the board, you risk it warping in that direction as that part of the board now dries out.
The Iroko I had a bunch of was pretty bad at this and the Ash I had was REALLY bad at this. The Ash was where I really learned how bad warping can be. I had laid up a little board I was going to use for a sign. I didn’t know about letting it rest after each step. I came in the day after removing the clamps to start cutting it to final dimension and the board looked like a see-saw. I still have the board just cause it makes me laugh when I see it.
A friend of mine does high-end furniture. He built an entertainment cabinet out of solid walnut for a client. When he delivered it, he told them that he’d come back in a few months to adjust the doors as the cabinet finished settling. 3 months later, the client calls complaining about the doors not closing. He had to re-explain everything he said the first time and then the next day he drove over and fixed the doors.
I have a small aquarium stand in my office that I made out of solid Oak. I used Oak plywood for the horizontal pieces, but the doors are rail and stile type from the solid oak. I had to adjust the hinges on them a few months after finishing too as the door gaps were no longer the same on both doors.
Those are just a few examples of it happening. I think it happens a lot more than we know, just not in ways we ever see it. If you ever build anything with in-set doors where you’re trying to maintain a tight gap, you’ll see it there.
The cutting board may end up having issues. It is my first cutting board. If you look close, you’ll see my squares don’t all make perfect corners either. If this one cracks mid-summer it won’t upset me. I’ll just go pick up more wood and make another one. This one already isn’t really the size I was shooting for, so I’ll probably redo it anyways. It’s all a part of learning.
My wife is working on the design for the laser engraving today. I’m hoping I can get it burned this weekend and finish it up.