Are you sure that when you change the codec that still mic 1/2 is used? I would rather think its a hardware issue and the mics are on the main board. As support is really really slow at the moment, I would strongly suggest to contact them via online Chat, opening hours change, currently it should be this
https://forum.fairphone.com/t/fairphone-support-no-response-received/115916/19?u=yvmuell