A lot of speculation that does end with this in the article:
"After discussing her case with experts, Matsapulina now believes her Telegram messages may have been compromised by a form of spyware. When she was told that a hacking device would need to be physically nearby to infiltrate her phone, a memory resurfaced: At times before her arrest, she had noticed an unmarked truck with a dome on its roof parked outside her building. She had even jokingly mentioned it to friends on Telegram. Now, she remembered, as the police were banging on her door that morning, she’d spotted the same mystery vehicle parked outside. By the time the police stormed her home, the vehicle was gone.
Matsapulina has since started using Telegram again."
Most messaging apps are vulnerable on the client side with spyware, no matter what E2EE exists along the way.