I have this same issue. I used CarPlay on two Outbacks ('18 and '21) and both times the phone was plugged in through the USB port. I now use CarPlay wirelessly with a "24 Ascent.
I make a call using CarPlay (after listening to music or a podcast with no issues) and the person answers the phone, we talk for 20 - 30 seconds, and then I can hear them but they cannot hear me.
To solve this, I hang up and call back and it generally works (although today it did not solve the issue), or I disconnect the phone and use the speaker on my iPhone and then they can hear me again.
Bluetooth and wireless are both turned on, the phone connects fine to CarPlay, but this happens often and is very concerning.