i've had a few like that.
a panelled door can, and is "supposed to," accomodate shinkage by the panels pulling fractionally out of the grooves (which is how a panelled door is made) in the thicker framing parts.
what happens, especially after many years of painting, is that paint gets into the grooves and firmly glues the parts together, so they can't move, and can't accomodate the shrinkage.
as shrinkage cannot be prevented, it usually cracks the paint or splits the panel.
when you're lucky, and it only cracks the paint, the fix is to scrape all the paint out of the join, so it can move freely, and apply just the thinnest smear of paint to hide the colour of the bare timber. It is less noticable on dark paint.
when the crack is accessible, it's possible to bump the two parts aside with the palm of your hand, brush in a little glue, and bump them together. A repair will not last if you don't remove the paint in the joints.
you might use a flexible silicone sealant, because the crack will open and close in future depending on the weather.
the panels will expand and contract widthways across the grain, not lengthways.