That sort of fading is due to the driver chip stopping driving the display - each pixel's capacitance then slowly discharges and every
pixel fades (dependent on variations between pixels and the last voltage it held).
Using a pot (as a potential divider) to provide a power rail is flawed - the voltage then will vary as the instantaneous current
demand from the driver varies, providing a very poor supply (unless bolstered with large decoupling capacitor).
It may also be that the device needs 5V, 3.3 is not enough?
I've had a poke about in the datasheets and something looks fishy: the TFT screen itself wants 3.3V, the adapter shield seems not
to have a voltage regulator (and is passing +5V to the TFT? Or maybe 3.3V to the LED backlight? Anyway it doesn't seem to
make sense to me, or the schematic is the wrong one? If the adapter shield is passing the Mega's 3.3V supply direct to
the TFT this could be an issue as the 3.3V supply isn't as beefy as the 5V supply on Arduinos - it might not be enough.