That’s a neat observation. Wood does contract and the grain can shift when it cools, especially in a dry environment. I’d suggest measuring the dimensional change at regular intervals—perhaps with a micrometer—and comparing it to the same board kept at constant temperature. That way you can quantify the effect and see if there’s any anisotropic behavior in the grain orientation. Keep the data consistent, and you’ll get a clear picture of how temperature alone affects the structure.