I have created a family group using the Apple ID A, and the account B is a member. However, I can’t download apps purchased by A in the device logged in by B. Today, I finally find out the cause. The Purchase Sharing setting in account B is incorrect, which is by accident set to share as A and I don’t know why.
To fix it, just ensure every family member has set to share purchase as themselves in the Purchase Sharing setting.
In case you don’t know how to download the Family Shared Purchase: