F
A reset will trigger a power cycle, which in turn resets the SPI connection to the e-ink display. The display has a controller built-in, which at this point is also power cycled, and most likely runs the initialisation of the screen. So when you press the reset button, the display thinks it needs to get into a "ready for new data" state, which is why it "fades".
If you want to work this around, I'd recommend storing the screen contents every time you write to the screen, to storage that is preserved during a power cycle (basically, dump that 200x200 4b array onto the flash), and make sure that your startup handling "updates" the screen with that content immediately after the device is initialised.