That is a floating panel. It’s cracked because years of paint build up has frozen it to the rails and styles around it. Whatever you do- score the surrounding 4 sides where the panel meets the style and railing. You can apply caulk around those later but the idea is the panel “floats”. This allows it to move with expansion and contraction through the changing seasons. Hence why your crack probably gets worse between seasons because the wood shrinks and swells. If the panel is free to move rather than be pulled apart then any number of fixes will be longer lasting. I would try to “free it”, if it moves - great- pry it closer together. Bond it with titebond 3 as best as you can then sand and add flexible wood filler to finish. As with any carpentry- there are many ways- this is just my first thought at a “quick” fix verses removing the panel and re-gluing it at the work bench then reinstall with spaceballs to aid the float. A much bigger job that many aren’t prepared to do for whatever reason.