I’m a healthy girl, and every morning I make myself oatmeal for breakfast using soy milk. It ends up being very tasty, and has a good texture, just as if I had used regular milk. However, yesterday I made it with chocolate soy milk, and it just ended up bad–the texture was awful, the taste was terrible, and it just looked funny. I cooked it for the same amount of time, in the same microwave, and used the same brand of soy milk, just different flavors. There’s no oatmeal difference between regular and vanilla flavored, so why should chocolate make such a big difference?
When I purchase regular, cow-type chocolate milk at the grocery store (for my son, or course, not for me!) I notice that it is artificially enhanced with a thickener of some sort. Apparently chocolate milk is supposed to be a little slimy. I wonder if this might be true for soy milk as well, and, if so, if this isn’t the coagulant that’s disturbing your oatmeal.
I don’t know what they add and I don’t have any chocolate milk in the house to check the label. I think they use a similar product sometimes in skim milk to make it have a creamier, less watery texture. It seems logical to me that a substance that provides thickness in a cold liquid might very well act as you described when cooked.
If you have any Nesquik or similar you might try making chocolate milk with it and cooking it. That might work, in which case it would be something besides the chocolate that is causing the problem. And I would be proven right. And you would send me money.
A note of warning – if you’re having oatmeal for breakfast, especially if made with soy milk, you should never sneeze down your shirt. Trust me.
Yes, they add carageenen (comes from seaweed) to a lot of chocolate milk, not just the soy milk. It helps hold the cocoa powder in suspension while it’s in transit between the factory and your house, for one thing. You know how when you make Nestle’s Quik, you have to keep stirring it up 'cause the chocolate all sinks down to the bottom? The manufacturers of chocolate-milk-in-a-carton-for-people-who-are-too-dim-to-make-it-themselves (present company excepted, TYGirl 
), knows that consumers won’t think to shake it up before they open it, and then they’ll be annoyed when they get down to the bottom and there’s all the chocolate down there.
And their chocolate milk food chemistry formulas are carefully calibrated to remain stable and recognizable as “chocolate milk” no matter how much the truck jiggles them around, but they aren’t designed to be used for cooking purposes, hence the weird oatmeal results.