If this is an accurate description, then I don't see it. I called a 5 WR smash play with 2 slot guys running corner routes.
I called a 3-4 pinch which rushes 7 (although I hot routed them all to spy so I didn't have to worry about a pass rush). The FS lined up across from his man, but like you said, the SS stayed in the middle of the field.
Normally I'd leave him there, but to test I moved him to line up across from his man presnap.
I took control of another defender and the SS stayed put.
I snapped the ball, and while the corner route was open (just like curls, outs, and ins frequently are against man) the SS most certainly didn't "sprint to the middle of the field, ignoring anything and everything, THEN go chase after his guy." He covered the corner route the same way as the FS that I didn't have to move pre-snap.
Here is how it looked for me. Is this not what you're seeing?