Recording audio (direct feed into iPhone) with video

Ok; am thinking this is an issue that others may have already reported.

I took delivery of a new 16 pro max a few days ago. An upgrade from my old 11 pro max.

I record and video/stream dj sets for social media and soundcloud. On my 11 pro max I did this by sending the output from my mixer into an audio interface (Art Cleanbox Pro). The sound was then sent from that, via a 3.5mm RCA, into my phone using a Rode TRRS connector into a lightning adaptor - into the phone.

On my 11 pro max this worked brilliantly. I assumed that for it to work the same on my new 16 pro max, all I needed to do was purchase a TRRS to USB C adaptor. Have done this and tried it out.

At first the sound was very quiet and muffled - but I had kept the sound levels on my mixer and Cleanbox Pro at the previous settings that worked for my 11 pro max. Fair enough - I increased the levels. Got them to the desired volume, ensuring they’re not too high as to distort. But the sound is still very muffled - to the point where it sounds almost underwater, or recorded in a large room. Definitely not clear or of a quality to be able to use.

The phone is picking up the direct sound input - I test this by turning my speakers up/silent. This makes no diff to the finished video, so I know the USB C input is sending audio into the phone.

Have checked out solutions online - have made sure my camera is recording stereo (not spacial) etc. Is there anything else I should be doing in settings?

Reading others’ comments I’ve seen there seems to be an issue with the 16 pro max recording direct sound input when recording video - is this correct?

This is massively inconvenient - recording/streaming music video is the biggest use I have for my phone.

Is this a known 16 pro max issue and can it be fixed?