Sounds more to me like a raw material problem. I can't imagine simply
dropping a plug would open up a longitudinal crack of that magnitude in
the case - or do I have the wrong image? A picture would help.

If the spark plug case is made of a free machining resulfurized or even
leaded steel alloy, these alloys are somewhat prone to having stringer
defects in them. On the other hand such steels are also brittle at
higher temps & I wouldn't think would be appropriate for sparkplug
useage.