There must be a bug somewhere, because for the code below I got:

const payload = {
        username: 'łukasz',
    }

    console.log(payload) // -> {username: "łukasz"}

    let frontToken = Buffer.from(JSON.stringify(payload)).toString('base64')

    console.log(frontToken) // -> eyJ1c2VybmFtZSI6IsWCdWthc3oifQ==

    let tokenInfo = JSON.parse(atob(frontToken))

    console.log(tokenInfo)  // -> {username: "łukasz"}