As the plugin documentation from within Jenkins states, when username with password credentials (for build auth) are selected as default for Bitbucket Instance in Jenkins configuration, users are not required to select them and the "global" ones will be used by default. However, leaving the corresponding field for credentials (for build auth) empty in a Multibranch Pipeline Job configuration, results in authentication failure in Bitbucket Server. The repository cannot be scanned, because repository cannot be accessed.
This means a core functionality is broken with current plugin version (see environment description). Blocks all users to create a job to checkout from Bitbucket themselves.
As the admin privileges are required to use the credentials, Jenkins admin needs to select the correct credentials for each job configuration.