Hi, I'm holding on porting JTAG2UPDI for the 32U4. The 32U4 (leonardo) is used for programming ATtiny via UPDI. While receiving data via USB (mapped to COMx) the 32U4 cannot send feedback to the Arduino-IDE (checking comport fails). Maybe a handshake issue on USB->COM? There's no way to disable DTR/DSR, RTS/CTS in the IDE.
When activating the COM-port on a terminal prog (like Realterm) after programming failed all bytes ment as feedback for IDE are sent.
JTAG2UPDI works with 328P devices, so they don't seem to care about handshake and send feedback data to IDE.

Update 2020年02月17日: Issue appears on Windows10 Arduino 1.8.10 (and later), works fine using Ubuntu1804-64 and Arduino 1.8.12