Users can’t report it because there is no way to tell for them
Atleast the one who breached can tell? no telegram users data have been seen on dark web yet, no person/org have claimed to get any vulnerability in their system. Also if its that easy to breach why govt’s keep banning telegram for not giving them userdata? despite telegram is the biggest app where most terrorist orgs operate, hub of piracy and illegal things, you can call it “public” darkweb.