The current Mcufriend pcb is badly designed. The backlight is powered by the wimpy Uno 3.3V pin.
I suspect that the working voltages are different between using USB and external jack. Simple enough to check with a DMM. e.g. measure the 5V pin and measure the 3.3V pin. Compare with the voltages obtained via USB.
Anyway, it is simple enough to tweak the Calibration values.