Here's what I found:

Over the past year, numerous dissidents across Russia have found their Telegram accounts seemingly monitored or compromised. Hundreds have had their Telegram activity wielded against them in criminal cases. Perhaps most disturbingly, some activists have found their “secret chats”—Telegram’s purportedly ironclad, end-to-end encrypted feature—behaving strangely, in ways that suggest an unwelcome third party might be eavesdropping. These cases have set off a swirl of conspiracy theories, paranoia, and speculation among dissidents, whose trust in Telegram has plummeted. In many cases, it’s impossible to tell what’s really happening to people’s accounts—whether spyware or Kremlin informants have been used to break in, through no particular fault of the company; whether Telegram really is cooperating with Moscow; or whether it’s such an inherently unsafe platform that the latter is merely what appears to be going on. ... Elies Campo, who says he directed Telegram’s growth, business, and partnerships for several years, confirmed this general characterization to WIRED, as did a former Telegram developer. In other words, Telegram has the capacity to share nearly any confidential information a government requests. Users just have to trust that it won’t.

https://www.wired.com/story/the-kremlin-has-entered-the-chat/