Mophie case protection against impacts on case corners

Dropped my iPhone 6 encased in an iPhone 8 Mophie battery case from a height of 3 feet, and it must have impacted floor on top corner of case, as 2 screen cracks now emanate from the top corner of screen. I have dropped this phone encased in a cheap flexible protective case dozens of times over last five years, and never damaged it. No damage to the case, but now I wonder whether the Mophie case is supposed to be able to protect against such drops of an iPhone 6 or an iPhone 8 encased in the Mophie battery case. Does anyone have any information about what height of a drop, impacting on top case corner onto a hard surface an iPhone 6 or iPhone 8 screen can survive without damage?