Currently, editing a ContentType with unordered FieldDefinitions (position=0) is allowed by the repository. However, viewing the same ContentType throws an InvalidArgumentException:

Argument 'position' is invalid: '0' is wrong value in class FieldDefinition, an integer strictly greater than 0 is required.

This exception is thrown in eZ/Publish/Core/Persistence/Legacy/Content/Type/Handler.php at line 285.