The audio is not received (technically not even streamed) by the call participant(s).
The audio is received and heard by call participant(s).
Due to Telegram Team investigation, it seems to be something not related to the app. It seems that Android API from the ROM mounted in the involved devices, is simply broken. Or is possible that is intentionally forbidden to extract audio from other apps. In this case, this is a manufacturer's decision.
Any Sony, Xiaomi, OnePlus, Oppo.