My unit had the same problem (see attached images).
I returned it to the shop and 2 days later they called me that the official examination found no clue of water damage or whatsoever, but they insisted that the problem the watch's 'feature' which is there to show the time dimly. I think they meant the always on display, but this is clearly image retention, as the image above was taken after charging on a dark gray image from gallery.
I already ordered a BT version from another shop (I had the LTE version, but BT is cheaper and being able to do emergency calls seems not very useful for me, as I was using 'dumb' watches in my entire life). I hope the first shop will send me the refund soon and the second unit will have no problems.