Microwave heating starts from the inside, so when the whole steamed bread is completely heated, the water in the center will be exhausted.

I think the principle has been clearly stated above.

Then I'll tell you the little coup of microwave oven heating steamed bread without hardening center:

Fill our usual dishes with 1/3- 1/2 water.

Then wrap the plate with plastic wrap.

Put steamed bread, rolls, etc. Just put it on it and pull it.

I tried, and it felt good.