When in a group audio call with more than 5 people, only the first five people to unmute can be heard by the others on the call.
Any additional people to join and unmute will not be heard, even though the interface shows them as unmuted, and the audio bubble pulsates on their local end as if they are being heard.
I asked a question on Reddit and was encouraged to report this as a bug, since this is not documented or expected behavior. (https://www.reddit.com/r/Telegram/comments/mxau8g/q_only_5_people_can_be_unmuted_at_a_time_limit_or/)
This bug affects Telegram iOS, Telegram Desktop on Linux, and Telegram Desktop for Windows, at least.
Telegram iOS 7.6.2 (20561), iPhone XR, 14.4.2