So the problem has nothing to do with the Arduino, it's the port expander.
What chip is it?
I obtain the same problem if i connect an lcd display I2C to arduino...it activate baklight for some seconds..
If the i2c lines are pulled up then no commands or rubbish should go to the LCD, therefore it will do nothing. Normally the back light is a separate connection. Are you saying that this LCD has control over the backlight using i2c commands?
Rob