Apple has AirPlay suspended or canceled when the iOS Device accesses internal sounds. Once the call is done or the app is closed, AirPlay mirroring may resume, depending on the length of time mirroring was suspended.
The iOS Device may only access AirPlay or internal system sounds. It cannot access both.
For specific apps that suspend AirPlay, you can access Settings>Privacy>Microphone and toggle the Microphone option off for that specific app. It will no longer have access to the iOS Device’s microphone for speech to text, but mirroring should continue while using it.