Thanks for your reply and suggestions 
I’ve just tried Open Camera and Audio Recorder. The audio on Audio Recorder is very good with hardly any loss of quality and certainly no ducking/automatic noise reduction. However, the results on Open Camera seem pretty much the same as the standard camera app.
My wife ordered a Fairphone 4 at the same time as me and we tried using her phone in the same way in case it was my phone that was at fault, but hers returned the same results: a very sudden reduction in volume level after approximately 1 second of a video where the background noise is predominantly road traffic (i.e. assumed to be unwanted noise).
Generally, the sound quality that accompanies any video is VERY bad compared to the Audio Recorder app (unless the noise level is high), or a dedicated handheld recorder such as a Tascam or Zoom, which really surprises me. I wasn’t expecting this aspect of the FP4 to be fantastic, but was certainly expecting more than this 