That is functioning as intended. Using the button above the license plate only releases the latch. The power door functions are triggered by the button next to the steering wheel and by the key fob only.

 

What mnewxcv is true, but maybe the gas struts on the gate are wearing out. Lifting the tail gate manually is somewhat heavy, but if the struts are almost worn out, it feels heavier.

When the gas struts are fully worn out, the tail gate might not open by itself and when it is open, it may not stay open.

 

I did just notice this line in the rx400h and rx350 owners manuals:

Power back door: Assistance of opening with door opener: OFF ON

This is a Lexus customization feature which the dealer can change, for a fee. I am not entirely sure what that does, if it uses power to raise the tailgate when you manually unlatch it or not. However, I do have the techstream software to change that setting and can try it tomorrow and report back if you like.

 

I went ahead and tested the setting with techstream (lexus dealer customizable feature) and be switching power back door assistance from OFF to ON, the power door opens when you press the button above the plate. If your vehicle has always been serviced at the lexus dealership, they may change this for you for free with your next service. However, it is not uncommon for them to charge a fee for changing these settings. Speak with your service adviser.

 

 

Yeah it is one of Lexus customizable features which Lexus will set for the first owner at the first service for free. A list of what they can change is in the manual I believe.