Is it a leak if it's a necessary technical part to a functionality?

The main issue is that it's not obvious to non-technical users. They can't asses what sharing IP address means either though.

The reason Telegram leaks a user’s IP addresses during a call is that, by default, Telegram uses a peer-to-peer connection between callers “for better quality and reduced latency,” Telegram spokesperson Remi Vaughn told TechCrunch.

“The downside of this is that it necessitates that both sides know the IP address of the other (since it is a direct connection). Unlike on other messengers, calls from those who are not your contact list will be routed through Telegram’s servers to obscure that,” Vaughn said.

To avoid leaking your IP address, you have to go to Telegram’s Settings > Privacy and Security > Calls, and then select “Never” in the Peer-to-Peer menu, as shown below.

Telegram defaults to using p2p for calls, for contacts only.

It's not a thorough privacy default, but otherwise seems fine to me. If you want p2p it needs to be enabled, and if you don't it needs to be disabled. No-contacts and no-calls receive no IP.