Cognito Generating UUID V7 instead of UUID V4

Cognito is responsible for generating an arbitrary unique string for the sub for its user pool. You can't control (or make assumptions) about the format. From https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-settings-attributes.html#cognito-user-pools-standard-attributes

sub

The sub attribute is a unique user identifier within each user pool.

See also https://repost.aws/questions/QUfWN5qGq8R5WsocQaP1A9Dg/cognito-sub-attribute-seems-to-be-no-more-a-valid-uuid