I'm not the chemist so I am just making a WAG here: Your chlorine level is so high it's finally bleaching the OTO. But I do not know if that is possible. Deep brown with OTO means about 40-50ppm. But after 2 drops? I just don't know.
Now, when you say it reads 4 when you mix 2distilled to one pool, does it read "4" on the scale? Because that means it is "12" (1:1, double the reading, 2:1, triple it and so on) If so, I'd keep increasing the dilution to 3:1 (16), 4:1(20), 5:1 (24), etc. What have you got to lose?