If you are receiving offers and messages from people whose items you have favorited, even though your setting « Send a notification to members when I favorite their items » is disabled (check here: https://www.vinted.fr/settings/privacy), then it is indeed a Vinted bug.
=> You must complain to Vinted so that this is escalated to their coding team. This is a privacy setting, so it is important that they respect your choice NOT to notify sellers of your favorites.