"Shouldt all pins be driven low when display is turned off ( noDisplay() )"
I would think so - most devices are sped'ed to not have IO lines more than 0.5V above the Vcc line.
The pin can be damaged, or power could leak into other circuits and leave device partially powered.
FInd a way to bring all lines low before power is removed, and don't them go high before the Vcc line does.