[M.O.A.]

How do you carry, in which position?
To your right or left side, or in front or back?

I'm guessing that you carry IWB in the small of your back. Sitting in a car, sometimes shifting positions could explain the cracking. Have you tried carrying in front?

 

[M.O.A.]

Knowing everything you updated with, I'd also recommend a hybrid leather + kydex or nylon combo for strength, durability and flexibility. The leather would 'breath' enough to prevent cracking, imho.

Maybe Crossbreed or similar could use leather backing and outer wings / clips with the center Kydex shell suspended between, rather than affixed to, the wings. Best of both worlds.