serverless: Adding custom attribute to Cognito user pool removes Cognito trigger lambda

When changing an existing Cognito user pool, by adding a custom attribute, an existing lambda trigger is removed (even though it is still set as a trigger in serverless.yml). And by removed, I mean that in the Cognito user pool settings, it has set the trigger back to None (the lambda function itself is still deployed).

I have an existing, deployed Cognito user pool, and a lambda PostConfirmation trigger, with the event set as such:

    events:
      - cognitoUserPool:
          pool: my_user_pool 
          trigger: PostConfirmation
          existing: true

I update my Cognito user pool to add a custom attribute, e.g. in the Resources section I’ve updated the user pool to look like (note the Schema section at the end, which is where the custom attribute is defined):

Resources:
  MyUserPool:
    Type: AWS::Cognito::UserPool
    Properties:
      UserPoolName: my_user_pool
      UsernameAttributes: # use email as username/login
        - 'email'
      AutoVerifiedAttributes:
        - 'email'
      UsernameConfiguration:
        CaseSensitive: false
      Policies:
        PasswordPolicy:
          MinimumLength: 8
          RequireLowercase: true
          RequireNumbers: true
          RequireSymbols: false
          RequireUppercase: true
      AccountRecoverySetting:
        RecoveryMechanisms:
          - Name: verified_email
            Priority: 1
      VerificationMessageTemplate:
        DefaultEmailOption: CONFIRM_WITH_CODE
        EmailMessage: 'Thanks for signing up! Your verification code is {####}'
        EmailSubject: 'Your verification code'
      UserPoolTags:
        NatureDose: 'true'
      Schema: # Custom user attributes defined here
        - AttributeDataType: String
          Mutable: true
          Name: appID

I then deploy, to add the custom attribute. Upon doing so, the PostConfirmation trigger for my user pool has been removed. If I go into the AWS console and restore the PostConfirmation trigger, subsequent Serverless deploys don’t remove it. This seems to be completely associated with adding custom attributes. I’ve seen this happen twice now (I actually have two custom attributes, and in both cases, it removed the trigger upon deploy of that change).

sls deploy

Deploy command output is standard successful deploy, with no specific Cognito information, etc.

Installed version

sls --version
Framework Core: 2.43.1 (standalone)
Plugin: 5.1.3
SDK: 4.2.2
Components: 3.10.0