I've never seen launch mode work without max power enabled (on a P85DL). It instead gives the 'both pedals pressed' message on the instrument cluster. Do you know something that we don't?
From the manual:
To Activate Launch Mode
Before activating Launch Mode, it is recommended that the brakes are slightly warm by driving for a few minutes and using the brakes a few times.
1. Set the acceleration level to Insane or Ludicrous and enable Max Battery Power (see Controls on page 104).
2. With Model X shifted into Drive and at a complete stop with the steering wheel straight, fully press the brake pedal with your left foot.
3. While still pressing the brake with your left foot, fully press the accelerator pedal with your right foot, then release the accelerator pedal. The instrument panel displays a message indicating that Launch Mode is enabled.
4. Within eight seconds, fully press the accelerator pedal a second time to pre- load motor torque, then within four seconds, release the brake.
When you release the brake, Model X launches forward.