Denon DN-X1100 possible problem

Hi,

I got some strange problem and need to check if anyone experienced the same and if it could be the mixer itself.

When I have master leds in orange around +3 or +5, sound on certain tracks (Im using serato) becomes quietter, just like someone is turning volume knob down. Then when I just have a slight move of the fader on that track to down (to lower volume), sound returns to normal (raises up). I'm not really sure if its broken mixer (got it brand new 5 months ago), or something else fucking with me. I can reproduce on the same tracks, but not on all of them. Is it some kind of safety attenuator in there or is it just broken ?

When master volume knob is lowered to around 0 or below, I have no such problems.

Any idea ?