I had released the 1.3.0 version of Notion.Net client which has the critical bug as it implements the some breaking API changes introduced in Notion API version 2021年08月16日.

For those who have upgraded their package to 1.3.0 can do any of the below so you will not face issues when reading the response of Append block children API.

• Downgrade to 1.2.0
• Pass the API version as 2021年08月16日 in client option when initializing the client

We are planning to create a separate branch for 2021年05月13日 version of API's and solve any bug fixes and will release 2.0.0 version of package which will support the changes introduced in 2021年08月16日 API version.