AWS Cognito Custom Message not triggered when sign up with external idp (Google, Facebook etc..)

guys! I have the following case. I'm building auth flow with amplify lib and added the ability to signup users with Google as an Identity Provider. For the backend, I use a serverless framework to add triggers when the user signs up. Please see examples of triggers below:

customMessage:
handler: functions/auth/handler.handle
events:
  - cognitoUserPool:
      pool: userpool-dev
      trigger: CustomMessage
      existing: true

postConfirmation:
handler: functions/auth/handler.handle
events:
  - cognitoUserPool:
      pool: userpool-dev
      trigger: PostConfirmation
      existing: true

The issue I have is that when I sign up from Google my CustomMessage trigger is not invoked. When I do not use sign-up from Goole, it is triggered and sends the custom message. Do you maybe know why signing up with an external provider does not trigger the lambda function?