get some distilled water, or even better, pure water, add one dumpling in it, and put into a glass jar and cook in a microwave oven. We also need a thermometer to tell if the water is 100 degree. Using microwave on pure water, even when the water reaches 100 degree, the water does not boil, but The temperature is high enough to cook the dumpling. Now, the water does not boil, i.e, there is no current to bring the dumpling up or down. We should see the effect of what the hot air inside the dumpling will do. The reason for this experiment is to eliminate the current of boiling current which I suspect contribute a great deal in bringing the dumpling to the surface, not just the hot air in the dumpling alone. If so that explain why when cold water is added, the dumpling starts to sink, because the reduce of hot current raising up.