Welcome to the Developer Forum! As of Web Meeting SDK v2.3.0 and higher, you can generate your signature using SDK App type credentials. You will want to migrate JWT to the SDK credentials. Can you try with the SDK credentials and let us know the results?
Are you seeing this behavior only when using your Heroku app? I can confirm the behavior is working as expected on my end. As a next step, the first thing I’d do is test if the same behavior persists outside of your App. To test the expected behavior against what you are seeing, you can use our Sample Web Meeting SDK along with the Zoom Meeting SDK Sample Signature Node.js app. This will help to identify the root cause.