It happens to me all the time.
It is worth noting that the red micro phone bar only appears when a BACKGROUND app uses microphone.
Something very strange is that when I quite the Instagram by sliding up on iPhone X, the red microphone shows up for a few seconds before changing to normal.
I guess is when Instagram is on the foreground, it listens to microphone “randomly”
That’s why it always asks for microphone access even if you just need camera to send a silent story.