Yes, that’s exactly what I am going to do: Using our zoom audio recorder. And that’s definitely not what I bought a new FP for.
I will make a request to the FP team, but seeing that this bug has been in their phones for years and is even in FP5, my hopes are limited…
What I will say in their defence, is that their support were actually super responsive and I received numerous, personalised responses from the FP team, not just from a chatbot or a redirection to an FAQ section. I’ll have to double check my FP4 again but I’m pretty sure the issue resolved itself, at least on my particular FP4, although I haven’t thoroughly tested it recently to know for sure. Really, if I’ve understood the issue correctly, it lies with the software that post processes and noise cancels/noise reduces audio once collected by the microphone, otherwise if the issue is with the microphone hardware itself, selecting “unprocessed” on the Open Camera app would potentially yield the same results.
I have the entry level Zoom H1n and I even attempted to connect it to my FP4 via USB C to use in place of the phone’s internal microphone, but for some annoying reason the audio then sounds as if it’s been EQd with a low pass filter set to around 10kHz i.e., the audio sounds muffled and there’s no top end. It seems FP is adamant that we aren’t allowed to record high quality audio and video simultaneously 
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No reply from FP so far, but I found a workaround for me using the dictating machine app (“Diktiergerät”, I don’t remember if it’s native, I don’t think so) :
-I start recording the (unprocessed) voice with this app.
-Then I switch to Open Camera and record a video without sound (is easy to turn off within the video option of Open Camera).
-The dictating machine app continues recording independently from the video recording, until I stop it manually.
So at last, I get both a usable video and audio file which I can merge during the editing process – without having to use my Zoom H1 (which I prefer over Zoom H1n).
This is actually a really smart workaround for your particular issue, good thinking! I hope you’re able to deal with the editing afterwards with as little work as possible!