Could be that is was cracked for a while and someone kept driving and due to time and vibration it busted out the rest of the way in one swift bang.
This , I have no idea how a neutral drop is responsible for that ???
There is no HARD mechanical connection between the back of the crank and the mainshaft of the transmission , total FLUID coupling inside the torque converter .
Neutral drops will only affect the trans internals , drive shaft and the rear end.
The many hours of running with that misalignment because one dowel pin was missing is what crack and ultimately broke that flexplate.
I'd be looking at the convertor snout and the bushing in the front drum for wear, along with changing the front seal.
Post up a picture of the converter snout ...