#[serde(flatten)] is implemented by internally buffering the visited data. Due to a bug / limitation in serde, the deserializer used to perform this buffering disrupts deserializer specific details, such as whether it's human readable or not (see https://github.com/serde-rs/serde/issues/1183). This can cause types that change their serialization behavior depending on the human readability to fail to round trip properly.

Originally filed on GH: https://github.com/mongodb/mongo-rust-driver/issues/530