When you talk about baking powder, if there is a considerable proportion of active dry yeast, and "the steamed bread is wrapped before steaming, it is obvious that the noodles are ready", it is likely that you didn't master it when you finally cooked in the pot. I once happened to see the looming chubby steamed bread surrounded by steam at the moment I opened the lid. I read it in the blink of an eye, and I thought a bunch of thieves were so strange.

Steaming steamed buns usually takes 10- 15 minutes. Turn off the fire after steaming, and don't open the lid at once! Imagine steaming 10 for more than 0 minutes, and there is no other gas in the pot except steam. Turn off the fire and open the lid immediately. Steam condenses instantly, forming a vacuum enough to crush everything in the pot, especially in winter, when the temperature difference is greater. You said that as soon as the lid was opened, the air went in. When there is a time difference, the air will enter from different angles, which is why you said "or it will be partially started". I have seen a formula for calculating the relationship between temperature, pressure and vacuum. In practice, there is often no such condition. So to avoid similar situations, one way is not to open the lid immediately after turning off the fire, and try to keep the temperature in the pot not too different from the room temperature. Usually wait 5 minutes before opening the lid, and the situation can be avoided. Just try it.