I thought it was clear, but I was wrong.
Telegram from website ( version 3.0.1 ), running in KDE under Unstable. Everything was ok until Bullseye came out and a tsunami of new packages arrived to Unstable. Since that moment the icon disappeared. Everything seems to work fine, except for that icon. No errors, nothing suspicious, neither in logs or running Telegram from command line. Just no icon.
Thanks in advance