Very glad your cutting problem is resolved. However, I am confused by this sentence,
"When I finally set it on a straight surface - like my kitchen counter - I could see that my ruler was making a shallow arch".
I retired a craft table that was a great space saver because it had two roomy hinged leafs. After the pressure from countless cuts, the leaves curved downward from the center (where the support was) to the edge of the leaves. It took me a long time to figure out why my cuts all curved. The cutting mat was curved to the shape of the top of the leaves, too.
New ruler, new cutting mat and using the dining room table solved the problem. Thank God!