First of all, it is not advisable to open the lid of the steamed cake right away, it is better to wait for the rice cooker to jump to the heat preservation, and then open the lid after ten minutes of simmering, so that you can effectively avoid the cake from shrinking back.
Additionally, if you don't beat the egg whites completely when whipping them, causing them to be unstable and prone to settling, this will also lead to the cake shrinking back when it cools down.
Also, if you put too little flour in the cake batter, it may cause shrinkage. Generally, it is recommended to use one bowl of flour with four eggs, and regular low gluten flour is fine.