Hello @Anonymous and thanks for reaching out. Out of the box, Azure AD B2C does not support storing user data in external data sources such as Azure Cosmos DB, which means your implementation is a custom one. The changes should be done in your application developed for managing the relationship between B2C users and Cosmos DB so that it targets the new data store.
Let us know if you need additional assistance. If the answer was helpful, please accept it and complete the quality survey so that others can find a solution.