After upgrading to the Pixel 4 XL from the Galaxy S8+, Bluetooth headsets microphones do not work in voice calls. This is the case for both a car Bluetooth system and Bose QC35s. I have attempted to reset the Bluetooth settings and reconnect the Bluetooth devices. This solves the issue for about one call then it no longer works.
Through Google searches I have seen that there are general Bluetooth connectivity issues on Android 10, but I have not found anything specifically related to this issue.