The problem is probably due to the fact that these applications only use a microphone on the handset to be used only for noise suppression. It seems that this problem has also been discovered on older models including OnePlus 5, OnePlus 5T and OnePlus 3. All applications are unaffected; Instagram for one apparently does not have this sound when playing back the sound recorded by OnePlus.
Some OnePlus 6 and OnePlus 6T users are wondering if a microphone problem is the reason why they have trouble activating Google Assistant when "Hey Google" appears. Fixing this issue may only require a software update, even though it is a third-party application.