Well, I managed to reproduce the problem with an original dvd (as they asked)
All the time they maintained that the problem was with the original recording, and that with an original blue ray (sic) or dvd it wouldn't happen.
Now that I have shown that it occurs with every source, they now sat that it's not a fault, they say that the panel cannot raise suddenly the brightness and power in an area without the phosphors being affected, as, for example, can do a led tv with local dimming, that's a completely different technology than plasma. (their words).
Except, it can, as the following video shows: it's showing a slide show of white square on a black background, with the square progressively bigger (5% of the screen, 10%, 15% and so on). When it shows the 10% square the background is perfectly black, showing that the panel can "raise the brightness in an area" without affecting the rest of the picture.
In spite of the evidence they don't acknowledge the problem and won't fix it.