One byte is lost when encoding a file into base64 in Golang

package main

import (
	"bytes"
	"encoding/base64"
	"fmt"
	"io"
	"log"
	"os"
)

func b(name string) {
	f, err := os.Open(name)
	if err != nil {
		log.Fatal(err)
	}
	defer f.Close()

	buf := new(bytes.Buffer)
	binval := base64.NewEncoder(base64.StdEncoding, buf)
	if _, err := io.Copy(binval, f); err != nil {
		log.Fatal(err)
	}

	fmt.Printf("%s\n", buf.String()[buf.Len()-5:])
}

func main() {
	b("soccer.jpg")
	b("soccer2.jpg")
}

soccer.jpg
soccer2.jpg

Output:

bodqhrohro@debian:/tmp$ go run base64.go 
nuNf/
nuNf/

The first file is identical to the second one just with the last byte cut out. They yield an identical base64 string. What's wrong?

I experience it with go1.15.9 and go1.18.3.

From the base64.NewEncoder docs:

> the caller must Close the returned encoder to flush any partially
> written blocks.

So:

binval.Close() // <- add this

fmt.Printf("%s\n", buf.String()[buf.Len()-5:])

see also the doc's example:

// Must close the encoder when finished to flush any partial blocks.
// If you comment out the following line, the last partial block "r"
// won't be encoded.
encoder.Close()