Flipping the display vertically doesn't work as expected

Hey everyone, first of all thanks for this cool library.

I'm trying to rotate my display by 180°, so flip it horizontally and vertically. I've found that your library provides both a flipHorizontal and a flipVertical function, but I either don't understand their intended behaviour or they don't quite work. Instead of flipping / mirroring the content, the display just clears with flipVertical. Here's an example sketch (run from an Arduino Uno) and a gif of the output (my display already took some beating...).


// Includes 
// ------------------------------------------------
// ------------------------------------------------
#include <SSD1320_OLED.h>
#include "graphics.h"       // stores all bitmap graphics

// pgmspace
#if (defined(__AVR__))
  #include <avr/pgmspace.h>
#else
  #include <pgmspace.h>
#endif

// Pins
// ------------------------------------------------
// ------------------------------------------------
#define OLED_CS 10          // Chip Select, OLED
#define OLED_RES 9          // Reset, OLED

// OLED Display Instance
SSD1320 flexibleOLED(OLED_CS, OLED_RES);

// Setup
// ------------------------------------------------
// ------------------------------------------------
void setup() {
    // Begin Serial for Debug
    Serial.begin(115200);

    pinMode(OLED_CS,OUTPUT);

    // Init OLED Display (Display is 160 wide, 32 high)
    flexibleOLED.begin(160, 32);
    flexibleOLED.clearDisplay();
    // Load Graphic
    //loadImage(macaque,sizeof(macaque));
    flexibleOLED.setFontType(0);
    flexibleOLED.setCursor(5, 5);
    flexibleOLED.print("Hello World");
    flexibleOLED.display();
}

// Main Loop 
// ------------------------------------------------  
// ------------------------------------------------
void loop() {
    delay(2000);
    flexibleOLED.flipVertical(true);
    loadImage(macaque,sizeof(macaque));
    delay(2000);
    flexibleOLED.flipVertical(false);
    loadImage(macaque,sizeof(macaque));
}

void loadImage(const unsigned char *image, int imageSize){
    flexibleOLED.setRowAddress(0);
    flexibleOLED.setColumnAddress(0);
    //Send the bytes from program memory to OLED display
    for (int i = 0 ; i < imageSize ; i++)
    {
      byte theByte = pgm_read_byte(image + i);

      flexibleOLED.data(theByte); //Write byte directly to display
    }
}

flipvertical (I didn't attach the graphics.h, as it just contains the macaque image as byte array, storing it in PROGMEM)

sparkfun / SparkFun_SSD1320_OLED_Arduino_Library

Flipping the display vertically doesn't work as expected #2