I will add my vote to the myth.
If we have a piece of wood that is at a uniform MC and is flat to begin with, as the case here it seems, and then a day later it is warped, we know that the moisture content has changed...100% certain. Further the moisture change is more on one side than the other.
Technical Explanation...
Now, consider an individual piece of lumber prior to being glued into the panel. Will this piece cup when it moisture changes? If it loses moisture, the bark side of the piece will indeed shrink more than the heart side, but unless the ring curvature is really high (meaning that the piece is cut from near the center of the tree- - the low grade part) the moisture movement difference between the bark side and the heart side is very, very small (seldom of a practical amount) if both sides gain or lose the same amount of moisture. We can possibly measure the difference in a lab, but it is not a practical amount.
However, if one side gains or loses 1% MC than the other side, this MC difference and the difference in shrinkage from side to side will cause cup. The amount of cup due to MC differences from face to face is much greater than the cup due to differences in shrinkage due to bark vs. heart sides. Note that the shrinkage difference bark to heart will be larger for thicker lumber, but even then it is small.
Appreciate that we often get cup when drying lumber, but in drying we have large moisture losses, so a wee bit of difference bark to heart times a 25% MC change will be 25 times more than with a 1% MC difference. So, even if there is a cupping issue in drying, with the small MC changes seen in service, the cupping in service will be very slight.
In the wintertime when plants are heated and therefore humidities are often below 30% RH or 6% EMC, it is common to see the top panel in a stack which is maybe 8% MC, being cupped upward. The wood is a bit too wet, it dries only on the exposed top surface and therefore the top surface shrinks, giving us cup. It does not make any difference on ring orientation, they always cup upward...every top panel. Obviously, better incoming MC control, so that there is no MC changes, will eliminate the problem. Otherwise, make sure that top and bottom have the same MC at all times.
"At all times" means during construction and after finishing...we urge people to finish top and bottom identically for this reason, especially with a large panel when even a little warp can be objectionable.
Hope this lengthy discussion helps provide understanding. I had the same question last month and so you will see this discussion in FDMC magazine.