[...snip...]


Again the only thing I keep coming back to is that the panel is a
single large but thin sheet of wood so that it may not take much
"differential" stress to lead to such cupping.

You know, your first statement about these panels implied the cupping
was recent; but if it is just that you are now noticing the cupping,
and it was there all along, then perhaps what you have are panels that
were formed long ago from wood that was not dry enough, or wood that
was reactive enough, to warp after the panels were formed.

Otherwise, it seems odd to have panels decades old all start warping
at the same time.

Regarding your other comment, that these are interior doors and the
panels are all indoors, so why is one side different than the other
.... I may be belaboring the point, but ... say you removed all the
finish from one side, and left all the finish on the other, then you
have a panel that will change humidity quickly on one side, and slowly
on the other, and the shrinkage/expansion on one side could be fast
with little change on the other, and you get a warp when the humidity
changes, and then the warp flattens out as both sides equalize
humidity.