I see that you clear the display before switching it off.
I did not clear the display and only used
Using the U8x8 library (part of U8g2). U8x8 does not use a screenbuffer.
I assume that your
disp.clearDisplay() clears the screenbuffer and
disp.display() copies the screenbuffer to the display?
It may be worth to do some further testing on this (clearing the display before enabling power-save).