I am using L432KB with mbed-os 6.15. I encounter a similar problem as yours.[Mbed-os 6.15 UART BufferedSerial and UnbufferedSerial not work on L432KB - #2 by JohnnyK](https://forums.mbed.com/t/mbed-os-6-15-uart-bufferedserial-and-unbufferedserial-not-work-on-l432kb
/17358/2)
Please let me know if you find a answer.
