According to the switch statement at https://github.com/wp-graphql/wp-graphql/blob/develop/src/Type/Object/RootQuery.php#L503, when querying a post preview via slug or uri, the resolver functions return early before checking the asPreview parameter and returning the revision.

This seems like it would be considered a bug because I would expect the same behavior regardless of which IdType is being used, also because I don't see any documentation that specifically explains why this would be intentional.