Boss Audio microphone very quiet with iPhone

I installed a Boss Audio BE920WCPA unit, and the microphone seems to be picking up audio at about 40% volume. I sound very faint to people on the other end of a call. Everything else works great—I can hear calls perfectly through the speakers. I think it’s an Apple issue, or a conflict with this particular model of head unit but I have no idea if it’s possible resolve it. I swapped out the microphone for another microphone (and tested both microphones by plugging them directly into a phone—they work great). With help from Crutchfield tech support, I connected with USB CarPlay only, wireless CarPlay only, and Bluetooth (without using CarPlay). I tried three different generations of iPhones, including one with an OS from about five years ago. I also tried bumping up the gain in the head unit settings, which didn’t seem to make any difference at all. Conveniently (sort of), I had ordered two of these units; one for my truck, and one for my wife’s car, so I even swapped out head units and the second unit had the same issue. I contacted Boss Audio, and they told me that there were no firmware updates for this unit. I asked a friend with an Android to test the unit, and the audio came through just fine (score for Android!), so it seems like it’s probably an iPhone issue. I’m wondering if others have had a similar issues with either the BE920WCPA or other Boss units—it if happened to me with two units, I wonder if it’s systemic.