I would like to direct attention to an often overlooked fact that WW2 soviet sharp nosed AP projectiles are not analogous in performance to sharp nosed projectiles of other nations and can either perform better or worse under certain conditions of attack, as these US tests with projectiles with reduced nose hardness show.
So lower nose hardness probably helps defeating sloped armor I assume, since you want hard noses to penetrate thick armor plates. A softer nose might deform instead of shatter, putting more energy into the plate instead of getting deflected away.
So it kinda makes sense to switch from rounds with not the highest T/D ratio penetration to blunt rounds that are better when they overmatch armor.