I think the function of the secondary microphone is for noise cancellation when making voice calls
Video recordings from HMD's camera app in Nokia 6 has always been mono. Even if the audio track in videos is stereo, it's just two channels with the same audio. I'm not sure if it used both mics previously but it only use one now.
The Open Camera app by Mark Harman can be set up to use both microphones (default audio source instead of camcorder, and stereo) but the result is poor on Nokia 6 since the audio then becomes heavily noise reduced.
With Open Camera I get better audio than in the HMD app with an external mic on a wired headset, especially in loud environments, but still only mono.
I haven't experimented with expensive mics, just a cheap aftermarket headset.
I'm not sure if HMD's developers made mistakes or deliberate decisions. It is not trivial to have exclusive access to the hardware in modern operating systems.