With a public share (add to my nextcloud) you can get the username of a nextcloud user. With
domain.tld/index.php/u/username
you then get the full user information also if you do not have an account on this nextcloud.
discussion: https://help.nextcloud.com/t/disable-public-users-contact-information-domain-tld-index-php-u-user/132006
Additional information:
Perhaps only a bug on Nextcloud 23.