When switching off the channel mixer you got 100% red and about 50% green and 50% blue values there.
Now in the channel mixer your G and B channels are lowered by a significant amount, using the red channel values. On that particular part, since R channel is 100% it will lower those values the most. In this case down to 0%, leaving only a saturated red patch (R100 G0 B0).
I guess you can only avoid this by dialing it back a little or compensate by boosting the G and B channels somehow.