Having Trouble with App Runner and RDS Access Issues

I deployed my app to App Runner with RDS MySQL, and I initially stored my database credentials in the App Runner environment variables. However, I usually access these credentials through SSM Parameter Store on my local setup. Now, I decided to switch to using Parameter Store directly in App Runner but I'm facing an issue when trying to access the deployed application. I'm getting an error that states, "Access denied for user 'user'@'ip.address' (using password: YES)." I can use Parameter Store without any issues locally, so I'm suspecting the problem is related to how App Runner is accessing it.