Can't remove, add or set \Seen

[tarach]

I'm trying to change \Seen flag in an email message but I'm unable to no matter how hard I try ... I'm using gmail account for testing.

require('dotenv').config();
const config = require('./config');

const ImapClient = require('emailjs-imap-client').default;
const client = new ImapClient(
    config.host,
    config.port,
    config.options
);

const getMessages = async (id) => {
    return await client.listMessages('INBOX', id, ['uid', 'flags', 'body[]']);
};

const printMessages = (messages) => {
    console.log(`Num of messages: ${messages.length}`);
    messages.forEach((message) => {
        if(message.uid === undefined) {
            console.trace(message);
            return null;
        }
        console.log('Flags for ' + message.uid + ': ' + message.flags.join(', '))
    });
};

const main = async () => {
    await client.connect();

    printMessages(await getMessages(7));

    printMessages(await client.setFlags('INBOX', 7, { remove: ['\\Seen']}));

    printMessages(await client.setFlags('INBOX', 7, { remove: ['\Seen'] }));

    printMessages(await getMessages(7));

    await client.close();
};

main().catch(err => console.error(err));

Output:

Num of messages: 1
Flags for 7: \Seen
Num of messages: 1
Trace: { '#': 7, flags: [] }
    at messages.forEach (/f/projects/sms-to-email/index.js:19:21)
    at Array.forEach (<anonymous>)
    at printMessages (/f/projects/sms-to-email/index.js:17:14)
    at main (/f/projects/sms-to-email/index.js:31:5)
    at process._tickCallback (internal/process/next_tick.js:68:7)
Num of messages: 0
Num of messages: 1
Flags for 7: \Seen

I was expecting mail that I'm trying to change to toggle being read/unread in the UI by font from bold normal and vice-versa but nothing happens.