This is because based on what users are finding out, it appears that the phone seems to get confused as to which microphone it should use. Apparently when used with third-party apps, the phone ends up using the microphone near the top which is typically used to help with noise cancellation and isn’t meant to be used for recording purposes.
Not all apps are affected by this and for the most part, it seems to affect apps like WhatsApp, Snapchat, and Facebook Messenger. Some users are also speculating that this could be one of the reasons why they are having difficulty triggering Google Assistant because the microphone isn’t picking up their voice commands.
So far there does not appear to be any kind of permanent fix for this issue. It is also unclear if this could be a software problem or a hardware one (presumably it’s software since native apps do not have this issue). Hopefully OnePlus will have a fix soon, although judging by reports that users have been making since the middle of last year, this is not a new problem.