Odd. Certainly it was never a problem when the cars were new and had full hub caps. The caps on the Newport must have become weak with age allowing them to rotate in the wheel.
Of course, you could kick a hubcap off when hitting a pothole or cornering fast...
Simple solution, get rid of the dang hubcaps. Which I know some like them and some do look good but I've always hated full hub caps. Ones that cover the lugs I don't mind, it's ones that surround the valve stem that I've always had issues with. Whether it's rotating and cutting the stem or simply packing the cutout full of snow and ice and thus not being able to fill the tire, they've always caused problems for me