I finally decided to figure out why my Raspberry Pi has been backdriving the LCD on my 3D printer. Apparently they put 2 different overvoltage protection diodes on the Vbus. One connected to 24V and one connected to 3.3V. The diode network on the 3.3V is connected all wrong.

As far as I can tell, the 5V from the USB doesn't even connect to anything...