bincode::encode_into_slice returns incorrect data

Question

I am using bincode crate(unstable version 2.0.0-rc.3) to perform binary serialization and deserialization of structs(trying to perform C memcpy of structs without padding into a buffer). While performing encode_into_slice in big endian format, the amount of bytes written into a [u8] buff is incorrect when compared the total size of the struct.

use bincode::{Decode, Encode};
use serde::{Serialize, Deserialize};

#[derive(Default, Deserialize, Serialize, Decode, Encode, Debug)]
#[repr(C)]
// size of struct is 10
pub struct Data {
    pub u: u16,
    pub f: f64,
}

fn main() {
    let data = Data {
        u: 1111u16,
        f: 1111.0,
    };
    let mut buff = [0u8; 16];
    if let Ok(processed_len) = bincode::encode_into_slice(&data, &mut buff, bincode::config::standard().with_big_endian()) {
        // prints value of 9 for smaller field values and sometimes 11 for larger field values
        println!("{}", processed_len);
    }
}

Cargo.toml

[dependencies]
bincode = "2.0.0-rc.3"
serde = { version = "1.0.195", features = ["derive"] }