Been a woodworker most of my life, and the stuff has a mind of it's own.
Kiln dried is good, but I've even seen that stuff warp and twist and cup and,....well, you get the picture.
I've had some spruce, white pine, and red pine in the side yard, stickered, for over 2 years. Not so much sun, but lots of wind.
Stupid me, thinking it should SURELY be dry by NOW, decided to use it as trim in the stove room. Nicely matched, mitered corners, became nicely separated mitered corners in about a day.
I'm now taking the just under 5/4 stock down to about 7/8's, then into the room it goes, to dry for a few days, before I cut it again. I'll cut it oversized, then dry some more, THEN install.
I hate mitered corners with gaps.
A nice solar kiln would be ideal.