Audio changes volume when recording video

Sometimes when I record a video on my iPhone, when I am speaking, the audio will start out very hot and then sound like it "compresses" everything causing the audio level to be drastically different and drop. Almost as if it is trying to reduce background noise. Has anyone else experienced this issue or know how to fix it?

When I try the same thing on my old iPhone 5s, it doesn't do this and keeps the same volume level throughout the video.