To display author avatar to public user, using the /web/image url with res.partner returns the placeholder since public doesn't have access. Using the url on mail.message will display the avatar according to the access right defined on res_model/res_id of mail.message. However this executes mush query to fetch the avatar image (1 per message, against 1 per partner).
Maybe this wasn't a bug, but since it is a regression, this deserves to be fixed ...
Jérome Maes
3304b31938