Using RotateFlip of bitmap class with pixelformat 1bppIndexed, image shifts 8 pixels left.

Question

//I create a bitmap to hold a 1 bit per pixel image:

format = PixelFormat.Format1bppIndexed;
bitmap = new Bitmap(width, height, format);

//Then I fill the bitmap programatically. ...

//I check pixels left and right using bitmap.GetPixel(x,y)...
//Then I mirror the image
bitmap.RotateFlip(RotateFlipType.RotateNoneFlipX);
//When I check the values again, the image is shifted 8 pixels (=1 byte) to the left.