Marking a mail as read (=seen) is not working

[pseisam]

I try to mark a mail as read (Flag: seen) by uid and it does not work. This is what I tried:

await imap_client.store(uid, '+FLAGS', '\\Seen')
await imap_client.store(uid, '+FLAGS', '\Seen')
await imap_client.store(uid, '+FLAGS', 'Seen')
await imap_client.store(uid, '+FLAGS', '\\Seen', true) #with all other variations of Seen from above

The response looks good, but it does not change anything: The flags stay empty

Response(result='OK', lines=[b'STORE completed'])

Any idea if this is a bug or wrong syntax by me?