The suspension will lower from Offroad mode to Normal mode when the speed exceeds 50km/h (verified on a 2007 TDV6 S).

This can cause problems especially when driving in sand, and picking up speed to get up a dune.  

When in Sand mode, and offroad height, on a 2007 TDV6 S, the behaviour is as follows:

•  At 40 km/h, a warning bell and message will tell you that vehicle will lower if speed is not reduced.
• At 50 km/h, the vehicle will then lowers back to  normal height.

This is obviously a safety and reliability issue - driving at too high a speed in offroad mode, will affect stability, and possibly puts a strain on the  CV joints.

However, if you do not pay attention, the vehicle may lower at an inconvenient time, causing you to get stuck when you bottom out and loose speed.