Hey Sebastien,
If I'm not mistaken, the red spot is a common documented effect of shooting into very bright spot sources (like the sun) on RED cameras due to the OLPF. I've observed it as well on my Weapon MG, generally the fix is unfortunately to open up a little more on the lens as far as I know. If you browse around on RedUser or some of the DSMC2 Facebook forums you'll find others with the same issue. The phenomenon seems to be unrelated to what lens you're using.
Hope it helps,
Andrew