Yes, what gthal says makes sense. Because everything happens so fast in 1st gear, the car wants to protect itself and not rev over 7k, which is what will happen if you change at 7k, no matter how fast our transmissions are! If you put the car in Sport+ and then manual mode, it will not shift up for you and just let you bounce off the limiter instead. However, this won't prevent over-rev in 1st because of the inertia of the car, even if throttle is taken away.
Your car is very smart y'see!
This video of the M550d xDrive accelerating illustrates our points of over-revving in 1st in manual mode. The driver is in DSC OFF so the car will not upshift itself and it actually goes past the limiter in 1st gear.
Skip to 0:38