I remember seeing this bug years ago. As Filippo mentions at the end, I'm still not sure whether to attribute this to malice or incompetence as per Hanlon's razor. I have not really followed what's up with Telegram lately, but I recall they had a rather brusque attitude towards the cryptography community at the time: "we have maths PhDs!", "Here's an encrypted message with no other context whatsoever: 0x459457453494530453409abc74f, $1 million if you can break it. No? Didn't think so!". To be honest, their consistent hubris at the time combined with (as far as I'm aware?) no other suspicious code (in the sense of backdoors, not just weird crypto) since, actually leads me to think it might genuinely be incompetence rather than a deliberate backdoor. I do think it's true that the security community can be a little outraged and not very welcoming to newcomers in the space if they get anything wrong: even Signal, pretty much the gold standard, receives constant (in my opinion, unfair) criticism for not being federated. Though, given the high stakes, I suppose this can be forgiven.