As a rule of thumb the first 3 threads of any fastener bear pretty much all the load. This is pretty much fastener design 101.
If a part fails at the beginning of the thread my first assumption would be that it is caused by a stress concentrator resulting from the manufacturing process of the thread forming. I assume that cylinder head threads are cut rather than rolled (bolts are rolled threads about 99.9% of the time). Rolling a thread is a process analogous to forging and produces a stronger thread and doesn't introduce stress concentrators from the thread cutting process.
There are formulae to calculate the thread strength. But, I doubt that its an issue because its worked successfully in aircraft engines for decades. I'd be pretty confident that the AD is the result of a material or manufacturing defect.
Properly torqued threaded fasteners do not come loose. Full stop. But typically we use fasteners tightened below the proper torque. Hence the need for Nylok nuts, lock wires, loctite, etc.
My guess would be that the shrink fit of the cylinder head to barrel is more to do with sealing than it is mechanical strength.