API page: https://api.drupal.org/api/drupal/core%21modules%21migrate%21src%21Plugi...
With the current example, the execution of migration will not reach the default_value plugin in the pipeline because of missing no_stub: true.
Without no_stub: true the migration_lookup will generate user and return the value.
Fixed
9.2
Primarily changes documentation, not code. For Drupal core issues, select the Documentation component instead of using this tag. In general, component selection is preferred over tag selection.
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