Issue with Custom Message Lambda Function in Cognito User Pool

I have encountered an issue with my "Custom Message Lambda Function" in AWS Cognito, which triggers after user signup. While the Lambda function is successfully able to set the "Email Subject," it is unable to set the "Email Message" as expected.

I followed the official documentation provided by AWS, specifically the "User Pool Lambda Custom Message" guide (link: https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-lambda-custom-message.html). However, despite following the instructions carefully, I am unable to set the desired email message.

To explore alternative solutions, I also referred to a Medium article (link: https://kim-jangwook.medium.com/cognito-cognito-custom-message-trigger-is-not-working-when-i-use-the-link-verification-type-4257c6e92e30). The implementation described in the article was working until 2021, but it seems to be outdated now.

I would greatly appreciate any insights or assistance from the AWS community regarding the reason behind the issue with setting the "Email Message" in the Custom Message Lambda Function. It seems that something may have changed in the Cognito service since the implementation described in the Medium article is no longer functioning correctly.