@GoreScarecrow Hi and welcome to the Forum
I have seen several posts started on here relating to headphone microphones not working. i.e. Microphone not showing from my bluetooth headphones and Settings of bluetooth earphones Neither seem to resolve in a solution, but they refer to bluetooth devices not USBC connection.
Just done a web search for the problem on ubuntu 18.04 and there are more of the same. So seems to be a wider problem. Also can’t seem to be able to find a real solution.
This is the best I have found so far, but not sure if there is a real solution there or not. https://askubuntu.com/questions/1085960/use-bluetooth-headset-with-microphone-hfp-in-ubuntu-18-04