The current Mcufriend Shield design is WRONG. It powers the backlight from the Arduino 3.3V pin which is WRONG. The original Shields had a 3.3V regulator on the display pcb which was powered by the 5V pin.
It is pretty simple. Look at the pcb.
The original pcb has an AMS1117 regulator. Both 5V and 3.3V pins are used.
The current pcb has an empty SOIC-8 footprint and no regulator. Only 3.3V pin is used.
David.