iPhone audio message quality

Every time I send an audio message, it will record at full volume and then get very quiet about a second later. You can see in the attached image that the quality of recording drops out fairly quickly. I speak at the same volume and distance from the microphone each time but this still happens. It doesn’t happen with any other apps (voice memo, camera, etc). It stays at a steady level when using the microphone on my headphones, but goes back to getting quiet when going back to the built in microphone.