Since the most recent update, the descent stage doesn't light its engines when the scenarios start. The MSL-like rover in the Gale Crater mission simply crashes into the ground at high speed (the fuel level also stars out low). The second rover in the Victoria Crater mission can be seen smashing into the crater in front of the player's medium rover. The attached screenshot is from the Gale Crater mission, right before crashing into the ground. Trying to manually control the descent stage does not work either.
The Space Program missions are not affected by this problem, but trying to control the descent stage on rovers with air bags is impossibly difficult. The descent stage spins and the direction the spacecraft is heading constantly changes. However, it is relatively easy to successfully touch down with a lander.