What's happening to the Nokia 808 Pureview @ ISO 1600?

I have got my hands on a brand new 808 which I've imported from India.

I have taken some marvelous shots with it and I can happily report that Nokia has addressed all major drawbacks of the N8. Here's one of those:

Nevertheless, one little thing is catching my attention.

It seems that the ISO 1600 setting presents us with some strange pattern around the lower part of the frame. All colours tend to blue and at the pixel level we can also distinguish a checked pattern. Sometimes it's also present at ISO 800. Look at these samples of mine:

ISO 400:

ISO 800:

ISO 1600:

Another set:

ISO 400:

ISO 800:

ISO 1600:

I was starting to think that it was a faulty unit, but this seems to happen with other units as well. Take, for instance, the high ISO test shots from phonearena's site:

What is intriguing me is that sometimes they come out quite normal: