22015 RX 350 F-SPORT:
At start activating the AWD drive button, dashboard lighted on AWD mode is activated and working, as you drive along I noticed the AWD light dissapear as yOU reach the 50kph speed, as you stop you can activate again the AWD button and see light on the dashboard. Is this normal for type of vehicle?

 

Its normal. After about 40 km/h the awd looses its effectiveness (for the purposes that most of us use the awd) so Lexus decided to design it like this Plus it saves fuel at front wheel drive configuration.

It also switches on automatically if it detects wheel spin at the front if speeds are less than 40 km/h.

 

From what I understand, the button turns on the "AWD Lock mode" and it's meant to be used when you get stuck in mud or deep snow, not for regular driving.
The manual says that the "AWD Lock mode" sends maximum power to the rear wheels, which I assume is 50% of the engine power.
Your AWD is active all the time and you can not turn it off, but the computer decides when to send power to which wheel.
You can try turning on the "Snow Mode", I use it when it snows and the car has excellent traction!

 

As stated above, AWD is automatic and no need to turn anything on. AWD "lock" will try act like a 4x4, but will soon disengage as it is not suitable for continuous driving.