then this line is not because data is left out but because pixels are moved to their correct position. The geometric distortions are corrected in the image, causing one (or both) border in range direction to be adjusted to the terrain topography.
This is completely normal and should actually happen for every image in this region. The stronger the topography, the stronger this effect.
Have a look at this tutorial: http://step.esa.int/docs/tutorials/S1TBX%20SAR%20Basics%20Tutorial.pdf
The same effect occurs there after terrain correction.