I have for a long period cooked raw rice directly with hot water in a porcelain bowl for 15 minutes in a microwave, totally open without a lid. The water does boil a little bit but not that severely, maybe because of the rice rich in starch which holds the water calm, so that the water doesn’t leak. It works perfectly despite the side which becomes a little bit dry. I usually resolve it by adding some water at the last 2 minutes, or removing the side.
While water boils only a little bit and doesn’t leak, milk does quite a lot, so water and milk must be different in some way.
I am concerned about whether the material, plastic, is problematic. Although disposable microwave-safe lunch box can bear up to 130℃, and a microwave can heat things maximally up to 100℃, any way, 15 minutes is apparently a quite long duration, so I am not sure if it would be harmful, e.g. releasing some chemicals, especially if it releases "secretly" that I cannot taste or smell it.
After use I will discard it. One box'd be used only once.
I've just come up with "instant soup" found in supermarkets, which should be heated as liquid for quite a few minutes in a microwave, and the plastic is disposable. I am not sure if normal disposable microwave-safe lunch boxes sold in dozens online could be the same type.